"Make every effort to give to [the poor], and do not feel bad about giving - for because of this, the Eternal your God will bless you in all of your endeavors." (Deut. 15:10)
How does God bless one who gives tzedakah? One might think it is like the following case:
A man walked through the marketplace and lost a wallet containing a $100. The following morning, he went to the market and found a $200 bill.
Yet this is not what the Torah is talking about. For this person, even though he found more money than he lost, is still upset that he lost the original $100. He thinks to himself: if I hadn't lost that $100, I would have now $300!
Rather, the Torah's blessing is like this case:
A farmer purchased a sack of grain, and carried it home through his fields. Not realizing there was a small hole in the sack, little by little the grain slipped out. By the time he reached his home, the sack was empty!
Some time later, the farmer passed through his fields. He was surprised to find that the grain he had unwittingly spilled on his fields had grown into a tremendous wheat crop. This man will not regret the seeds he lost. He realizes that if they hadn't spilled out over his fields, he would not have been blessed with a wonderful crop.
This is what the Torah promises. "Don't feel bad about giving" - for it is through this very giving that you will be blessed in your endeavors.
(Adapted from Mishlei Yaakov, p. 431)