Is it permissible to pay off a father's debt with money from bank interest, and if it is forbidden, can it be paid from lawful money? And what should a person do with money from bank interest if it is unlawful?

Bank profits are considered usury (riba), and it is forbidden to spend them on oneself or on those one is obligated to support. Rather, one must dispose of them by spending them on charitable causes.

It is permissible to pay off a father's debts with these funds under two conditions: that he is unable to pay, and that the debts were incurred for a purpose permissible by Islamic law. If these conditions, or one of them, are not met, then it is not permissible. Paying them off with lawful money is more preferable.