Is it obligatory to pay back what was spent from the usurious interest that the mother took from the deceased father's money by ruling of the Hasbi Council, and is it sufficient to pay back the principal if the spent interest is greater than it?

Bank interest is forbidden usury. Whoever has engaged in it must repent immediately and withdraw from it, and dispose of the forbidden interest by spending it on charitable causes and public welfare. If this money devolves to poor heirs, they may benefit from it because they did not acquire it themselves. If you were poor at the time these interests were spent on you, there is no blame on you. However, if you received them after you became self-sufficient, you must dispose of them.