Is it permissible for a father to give his son-in-law money from usurious funds that he donates, to pay off his debt, without informing him of the source of the money?

There is no harm in disposing of usurious interest by giving it to the poor husband of one's daughter, or to a debtor unable to repay, and there is no need to inform him of the true nature of the money; because it is lawful money for him to possess, and it is not forbidden for him.