To whom should the money left by the deceased be handed over, which is owed to the borrower, given that his heirs are: a son from his deceased wife, and another wife?

What is obligatory upon you is to pay the amount to the person who possesses the deceased's estate so that it can be divided among all his heirs. The wife has a right to it, and the child also has a right, after the deceased's debts are paid and his will, if any, is executed, as Allah the Almighty says: ﴿after any will he may have made or any debt﴾. Thus, what is owed by you is to be added to the deceased's estate and not given to one heir to the exclusion of another.