Is it permissible for a mother to pay off her son's defaulted loan, which is based on Riba (interest) from the bank, using the usurious bank interest she previously got rid of, knowing that the son is unmarried, lives with her, and is partially dependent on her for his expenses?

It is not permissible to deposit money in interest-based banks when a sharia-compliant alternative is available, and the intention to dispose of the interest does not make it permissible. The forbidden interest should be spent on the welfare of Muslims, the poor, and the needy. It is not permissible for the recipient to spend it on themselves or their dependents unless they are poor to the extent of their need. It is permissible to give it to a poor relative whose maintenance is not obligatory upon one. If the aforementioned brother is poor and in need, there is no harm for the mother to give him some of that interest for his need or to settle his debt. Sincere repentance from usury is obligatory, involving cessation, remorse, and a firm resolve not to return to it.