How can I pay off an old debt whose exact amount I do not know, especially since the creditor has forgiven me if I am unable to pay?

You can ask the owner of the money about the amount of his wealth, and if he has forgotten, then take precautions until you pay what you owe or he forgives you. You have done well in seeking to free yourself from people's rights before death, for the matter of debt is great, according to the saying of the Messenger of Allah, peace and blessings be upon him: "The soul of a believer is tied to his debt until it is paid off." Its meaning is that its matter is suspended; it has no ruling of salvation or perdition until it is seen whether its debt will be paid off or not.