Is it permissible for a father to help his son repay his usurious debt by lending him money, and then for the father to collect the repayment of this loan from the rent of his son's house to him, knowing that this might lead him to favor this son over his other siblings who also rent from him?

Borrowing from usurious banks is a grave sin. Whoever has done so must repent, feel remorse, and resolve never to return to it. It is not obligatory to repay previous debts immediately unless doing so would waive the usurious interest. If a father designates a loan to one of his children, this falls under the category of donation. Scholars have explicitly stated the disapproval of favoring one child over another in giving, unless there is a legitimate Sharia justification for it, such as if repaying the debts would waive the usurious interest.