Suppose you have got one thousand dollars in your pocket.  You spend it for some good cause. But that sort of sacrifice is also cause of your bondage. Now, suppose you have given to a poor man one hundred dollars to help. According to law of karma, you have given one hundred dollars to a poor man to help him. This means that the poor man has to pay you four hundred dollars in your next life, with interest and compound interest. And you will have to take that four hundred dollars. And suppose you are preparing yourself to conquer over the next life, but by contributing this one hundred dollars you are now bound up to take payment of four hundred dollars; therefore you have to take your birth. These are subtle laws.

>>> Ref. VedaBase => Bhagavad-gita 2.58-59 — New York, April 27, 1966