The Rambam (Maimonides) says that if all the money is not available to do all the mitzvos concerning Purim, then a person should spend what money he has on providing for the poor. Why?

There is a statement in the Talmud that if only one holiday would be kept after the redemption, it would be Purim. On all the holidays there is a commandment to be happy. But on those holidays we are happy so that we can better fulfill the actual holiday, sort of an assistance to the main command, which is to commemorate an event in history. Purim, on the other hand, does not mention G-d in its story, and the command is more about being happy itself.

So what is it that makes a person truly happy? Happiness is achieved when a hidden truth is revealed, and in this case it is a hidden truth about G-d. In the story of Purim, G-d's hand is in the background to help the oppressed, the Jews who were decreed upon by the king for murder. Just as this is the revelation of G-d's hand that makes us happy, we are made happy similarly when we emulate G-d and help the oppressed, the poor, by giving provisions for them on Purim. This is the deeper reason in the Rambam why he is of the opinion that giving to the poor is the most critical aspect of Purim.

Adapted from R. Sokol's sermon 2/27/99