Is it permissible to demand the money that was given to a poor brother as aid for his marriage if the brother later becomes wealthy and the giver becomes poor and in need?

You have done well in helping your brother, and he should do well by you, according to the Almighty's saying: "Is the reward for good [anything] but good?" If you gave him money as charity, then you are not allowed to demand it back, according to Ibn Qudamah's saying: "It is not permissible for the one who gives charity to take back his charity, in the unanimous opinion of all of them," and according to the Prophet's (PBUH) saying: "The one who takes back his gift is like one who takes back his vomit." However, your brother should help you in return for the favor and to maintain kinship ties.