Is it obligatory for someone who borrowed money from the deceased to return it to the heirs or to its original owner?

It is not permissible for the person from whom the father borrowed to return the amount to the lender; rather, he is obligated to return it to the heirs. The heirs are the ones who settle the debts of their deceased, even if the father had informed him of this or authorized him to repay the debt, because the agency is nullified by the death of the principal.