Charity Increases Sustenance and Protects Wealth
Cause and effect, are from the wisdom of Allah (S.w.T.). He has connected the cause and effect of things. For example, the person who uses good methods in planting his crops, will get a good harvest. Just the same way a person who eats and drinks healthily will have good health.
Allah (S.w.T.) too has linked spiritual and metaphysical causes to spiritual results. Allah states that the one who is beneficial to his relatives will have an increase in his life span. Also Allah states that giving charity removes evil. We human beings wonder how the cause and effect is connected, and what is the wisdom behind the incidences in life? Some cause and effects are easy to explain and the wisdom behind them is clear. On the other hand some of these phenomenas are hard to explain. However Islam stresses that giving charity increases sustenance. Here are a few traditions that stress on this fact.
The Holy Prophet (S) says: Attract sustenance by giving charity. 
Abu Abdillah (a.s.) says: Allah gives help to person equal in quantity to the charity a person gives.
Imam Jafar Sadiq (a.s.) says that: Everything has a key, and the key to sustenance is charity. 
Here below are some real life events that will illustrate these facts.
Effects Of Charity
One famous scholar, Sheikh Muhammad Hadi Al Amini relates that his father while acquiring knowledge in Najaf, Iraq, went to his hometown Tabriz. While he was in his hometown, he used to visit his relatives and friends.
Sheikh Aminis father didnt see an old acquaintance of his who was a poor and needy man. When he didnt see this friend, he asked about him and was told by people, that the man he was asking about was a rich man and no longer poor. Sheikh Aminis father decided to pay a visit to his friend.
While conversing with his friend he asked his friend the reason for the change in his economic position. The man replied that he had been very poor to the extent that he was forced to sell some properties that belonged to him, like furniture etc. By selling these things he had collected some money enough to start a business with.
As he was on his way back to his house with the capital, he saw a poor lady crying. The man asked the lady the reason for crying, and she explained that she was a widow with orphans who were hungry. The man felt pity on her, and on seeing that she needed the money more than himself he gave the whole amount of money to her. He returned home and spent the night depressed and worried for his family.
The next morning, surprisingly he was summoned to the house of a rich man. When he went to the rich man he was told by him that he had six thousand bags of rice which he feared were on the verge of expiring. The rich man suggested to him to buy the rice at a low price on loan from him. The poor man did so and sold the rice bags quickly. He took some more and in this way was able to make much profit. Soon he became rich and well to do. 
Indeed the Quran says that: The person who lends to Allah, a good lending will receive many times more. (Sura al-Baqara, 2:245 )
Help according to your capability!
Sheikh Abdul Aziim Al Muhtadi Al Bahraani in his book Qasas wal Khawatir relates that a beloved friend of his came to him in need of some money. Although the Sheikh didnt have anything to give this friend, yet he asked the friend to meet him at a certain venue so that he would give away the little he possessed, thinking of the saying that if you cant give everything you can give a little.
As the Sheikh waited for this friend, he came across a verse of Quran which says that a person should give charity according to his means, and that Allah doesnt burden a person more than his capability, and surely after difficulties there is ease.
The Sheikh saw that the verse was describing his position in respect to his friend, as the Sheikh didnt have much to give to his friend but he would help his friend according to his capability. As he took out an envelope containing the little he had, he decided to write the verse of Quran on the envelope.
As he finished doing so, he found his friend coming towards him happily and saying that he had obtained what he desired soon after the Sheikh had promised to give him something.
Allah increases tenfold!
I encountered this incident personally, and I will reveal it in detail.
A friend of mine telephoned me and asked me to collect some money for the commemoration of the martyrdom of Imam Hassan (a.s.) to help some needy people. I started to do so and shortly after I collected fifty dinars, while the amount needed was sixty dinars. In order to fill the deficit I personally added ten dirhams myself since the time was short and it was needed urgently.
Three days after the incident a friend of mine phoned me and told me that as he was looking into his account books he found that he owed me a hundred dirhams, and wanted to pay me back quickly. I was surprised and could not remember how he came to owe me this money.
However he reminded me, and told me that he used to pay me back in instalments, and after checking his books he found that he had not paid me the last instalment. Well, he paid me the money, and as I was thinking over the events I remembered how Allah promises in the Holy Quran, to increase a good deed ten folds. I remembered the amount of ten dirhams that I had taken out as charity.
Help to a Jewish girl!
A great scholar Al Haj Muiin Ashirazi relates that a friend of his Sayyid Darshooji who was a businessman said to him that his business was in a bad state and that his capital had been depleted and he was in debt. As he was in this dire state, a Jewish girl once came to him and informed him that she was getting married and that she was an orphan. She asked him to take pity on her and to sell her some home accessories at a cheap price.
Although the businessman was in a bad economic state, he decided to sell the girl the goods at a cheap price. Whereby the goods cost a hundred and fifty tumans, he gave her a discount of thirty tumans and sold her the goods at a loss.
Soon after that, he was offered some household commodities that were rare to be seen in the market at a very low price and with an additional advantage that they were offered to him on credit. Although he was hesitant to take on the deal, he did so and ended up making a huge profit, since people were in need of the items due to their scarcity in the market. In this way he recovered his capital and his economic condition improved.
Charity is key to sustenance!
Sheikh Kulaini in his book Al Kaafi relates the following tradition from Imam Jafar Sadiq (a.s.): Cure your sick by giving Sadaqah or charity, and remove troubles and mishaps by giving charity, and increase sustenance with charity. Charity banishes seventy shaitans from what is within ones beard, and charity goes unto the hand of Allah before it goes unto the hand of the needy. 
Our Holy Prophet (S) says that: We should give charity as it increases our property, and it makes the mercy of Allah befall on us. 
Once Abu Abdillah (a.s.) asked his son how much money he had. His son replied that he only had forty dinaars. Abu Abdillah (a.s.) asked his son to give it all away in the way of Allah. His son was hesitant and said that it was the only meager amount he possessed.
His father replied that: Charity was the key to sustenance.
Shortly after having given the amount in charity the Imams son got four thousand dirhams. The Imam (a.s.) said to his son: O son, we gave Allah forty dirhams and he gave us four thousand. 
The blessed harvest!
Mullah Fatah Ali relates that one of his close friends related to him that he had several farms. One year due to bad economic conditions people were suffering from starvation. This man decided to give the harvest of one of the farms to the poor. So he went to the mosque and proclaimed that people could benefit from this land, provided they were in need, and that they would not take from the harvest more than that which was necessary.
The needy went there everyday in great numbers and benefited. The owner of the farm was so busy with his work that he didnt follow up closely on the condition of that piece of land. After he had harvested all the other pieces of land, he remembered this particular farm he had given as charity, and ordered his servants to go and collect any dry grass, plants and seeds that may have remained.
To the great surprise of the servants, they found that there still remained a lot of crop that had not been harvested and consumed. On collecting the harvest, they all found that the harvest of that piece of land exceeded that of all the other pieces of land that the man possessed.
In addition to that, while it was a common practice to leave a land without planting any crops the following year so that it could regain the minerals it had lost, it was found that this piece of land had not lost its fertility at all, and it was possible to cultivate it in the following year. 
Indeed Allah increases for whomsoever He wishes. (Baqara 2:261)
Charity increases your knowledge!
It has been related from Mirza Khalil, may Allah have mercy on him, that he was not a physician, but he got the knowledge of medicine from a simple act of charity, as a gift of Allah to him.
He says that in his youth, he once visited the city of Qum that has the mausoleum of the sister of Imam Ridha (a.s.). During those days there was war between the countries of Iran and Russia, and there was lot of hunger due to bad and deteriorating conditions, especially when the cities of Iran were filled with prisoners from Russia.
He relates that his room was in one of the rooms of Dar ul-Shifaa, the top story of which constituted of a Madrassah and some rooms in which the visitors took abode.
Once Mirza Khalil went to the market to buy some bread and after a lot of efforts he got some. On the way back to his room he saw a Russian woman who was carrying a baby and was pale with hunger. When she saw Mirza Khalil with the loaf of bread she rebuked him, blaming him that the Muslims were not keeping the prisoners in a good state. Her words had a deep effect on Mirza Khalil and he decided to give the bread as charity. He then went back to his room feeling deep pangs of hunger.
Shortly after, a stranger knocked on his door to ask him where he could find a doctor, since the strangers mother was ill. Mirza Khalil asked him about the symptoms of the ailment, and when the stranger described to him about the nature of his mothers sickness, Mirza Khalil found himself automatically or involuntarily prescribing some medicines to the stranger.
When the stranger left to look for the medicines, Mirza Khalil was really puzzled, since he knew that he didnt have any knowledge of medicine, but he was able to prescribe the medicine. Soon after, the stranger came to thank him and tell him that the medicine had cured the sick mother. Also, the stranger brought food and gold coins as an appreciation to him.
After some days the stranger brought to him several people suffering from various ailments, and Mirza Khalil repeated the same act of prescribing the medicines to them, and they were all cured. This is when Mirza Khalil realized that this was a gift he had got from the Almighty in return for the act of charity.
Mirza Khalil became a renowned doctor without passing any exams except that of a spiritual nature. In addition he studied medicine and saw that he grasped its principles with ease and reached great heights in this new career granted to him by Allah. 
Charity to animals!
Imam Husayn (a.s.) saw a slave eating a morsel of food and then giving a morsel to a dog near him. The Imam asked the slave the reason why he was feeding the dog. The slave said that I feel ashamed to eat without feeding the dog.
The Imam asked the slave to wait until he returned, and went to the slaves master, and bought the slave and the piece of land on which he worked from his master. Then he freed the slave. Thus the slave achieved his freedom as well as becoming the owner of a piece of land due to his kindness to the dog. 
Half prosperity, half distress!
It has been related from Abil Hassan Al Mussa (a.s.) that there was a man amongst the Bani Israelites who saw a dream in which an angel informed him that half of his life would be prosperous and another half would be in distress. The angel asked him to choose whether he would like the first half to be in prosperity or the latter half.
The man woke up from his dream and consulted his wife. His wife advised him to choose the first half to be in prosperity, suggesting that Allah may have mercy on them later on.
The second night the man dreamt the angel again who demanded the answer from him. The man replied exactly what had been discussed by him with his wife. As prophesized by the angel the dream came true and the man was leading a very prosperous life with an abundance of blessings.
However his wife advised him to help his relatives, neighbors and the needy. Soon the first half of his life came to an end, and despite his expectation of problems and distress he found no change in his life, and still had all the blessings. He was puzzled and in this condition he dreamt of an angel who informed him that Allah thanked him for all the charity he had given and so decided to increase the blessings given to him.
A poet once said that if you wish to do good do so, and if you decide to sow a seed then sow a good one since a good seed will produce seven hundred more seedlings but a bad seed will only result in a bad outcome.
The famous poet Firdous says that: A good deed is for yourself and a bad one also returns back to you. 
Khums protects your wealth!
Sheikh Muhammad Ali Dakheel says that once he was in the market of Najaf when news was brought to the merchants of Najaf that their goods which were on the way to Najaf from Karbala have all got burnt. All the merchants decided to go to Karbala. When they approached Sheikh Muhammad Dakheel to go with them, he said that he trusted Allah to have secured his goods as he had paid Zakaat and Khums on it.
As the merchants were conversing they found a porter bring the property of Sheikh Muhammad Dakheel safe and sound without any damage. The porter said that his colleagues had refused to bring the goods at the price quoted to them but he had agreed to do so. 
Charity converts a Christian to a Muslim!
A poor man once came to one of the many Iranian cities, in a very bad state, with no money to buy food. As he walked through the streets of the city he remembered the tradition of our Holy Prophet (S), which says that if a person has a need and wants to ask something from someone he should look for a person with a pleasant face.
As he glanced at the people in the streets he found a man who fitted the description, and before he approached the man he asked people about the man. The people in the streets informed him that the man was a doctor by profession and was a Christian by faith. However the poor man decided to go to the Christian doctor, and asked him to check his body as he was suffering from some ailment.
The doctor asked him to sit and then checked his pulse. The doctor then asked his servant to go and get some food. The servant brought the food and the poor man ate till his hunger vanished. Then the doctor gave him money, and informed him that nothing but poverty ailed him.
The poor man thanked the doctor and left praying asking Allah to cure the doctor of a spiritual ailment and grant him guidance to Islam. As soon as the poor man had uttered this prayer when the doctor ran up to him and said that he had accepted Islam, and that the poor mans prayer had been accepted.
This shows that charity can give you a spiritual reward that is more beneficial than a material reward. 
Bihar vol 74 page 392
 Bihar Vol 76 Page 311
 Bihar vol 47 page 38
 Najafiaat Page 217
 Ref: Sura Talaq 65:7.
 Qasas wal khawatir page 280
 Bihar vol 96 page 129
 Bihar vol 96
 Bihar vol 47 page 38
 Al dhunoob al kabira page 197
 Jazaa al Amaal page 65
 Kunooz Al Hikma Page 280
 Jazaa Al Aamal Page 127
 Najafiat Page 134
 Jamiul Qasas Page 59