What is the ruling on the deceased's father paying off his son's debt, with the heirs being asked to repay him later?

Firstly, paying off the deceased's debt takes precedence over distributing the inheritance, based on the Almighty's saying: "after any bequest he may have made or any debt." Debt takes precedence over a bequest. If the deceased did not leave behind enough to pay off his debt, the heirs are not obligated to pay it unless they volunteer to do so.

Secondly, if the grandfather agrees with the rest of the heirs to pay off the debt on behalf of the deceased, on the condition that they repay him what he paid when they become able, there is no harm in that, and they are obligated to fulfill the promise. This is because breaking a promise is one of the signs of a hypocrite, and fulfilling a promise is a religious obligation. If he gave them the money to pay off their father's debt, they are obligated to repay the loan by the consensus of scholars.