How can one absolve oneself of the liability for a deceased father's debt when he did not forgive his debtor?

Rights must be returned to their owners, and one must repent from having squandered them or consumed them unjustly. If the owner of the right dies, then return his right to his heirs. If it is impossible to identify them or reach them, then give the right as charity on behalf of its owner. Ibn Taymiyyah stated that any wealth whose owner is unknown should be given as charity and spent on the welfare of Muslims. Al-Ghazali clarified that if it is impossible to return the wealth due to the absence of its owner or his death, it can be given as charity on his behalf; otherwise, you must multiply your good deeds and implore God to make Him pleased with you on the Day of Resurrection.