Is it permissible to pay the debt of the deceased husband's father to the insolvent brother of the husband, with the intention that it be a repayment on behalf of the deceased and a charity on behalf of the original owner of the money whose heirs are unknown?

Paying off debt is obligatory, and the debtor's responsibility remains engaged with it until he pays what he owes. If the heirs of the creditor cannot be found, then this money can be given as charity on behalf of the mentioned brother and his family or others, with the option for the heirs, if they are found later, to either approve the charity or reclaim their money.