What is the ruling on a son taking his mother's money without her knowledge when he is fulfilling her needs, and does he bear sin for that, and to whom does the charity of the donated money return?

It is not permissible for a son to take his mother's money without her permission, whether the amount is small or large. What the questioner is doing by keeping the remaining money or disposing of it without her permission is not permissible. He must repent and inform his mother of the matter. If she forgives him, then that is good and excellent; otherwise, he must return the amounts he disposed of.