The tractate of the Mishnah which deals with the laws of T’rumah seems to assume that everyone is in a position to give. Five categories are excluded, “the deaf mute, the idiot, the minor, one who gives from that which is not his, and the non-Jew”.
The deaf mute cannot communicate (at least in those days: medical science has now made life much easier). The idiot cannot understand what is going on. The minor is not yet ready for the mitzvah. The thief cannot use donations in order to whitewash himself. The non-Jew cannot be compelled to support Jewish causes.
But what about the poor person?
Their gold, silver and brass may well be their capacity to understand the need and to respond to it as fully as they can and to encourage others to commit themselves to the cause.
One Yom Kippur, a rich man wanted to stay behind on Kol Nidrei night and recite Psalms. The rabbi told him, “That’s for poor people, not for you. What you ought to do is to go home and get a good night’s sleep and resolve to give more charity in the future!”
The rich person must give of his means: the poor person can give in other ways.