Is what the father took from his grandfather's retirement salary after his death considered a debt upon him, and how can it be paid off?

What your father took of money in the manner mentioned is consuming wealth unjustly, and he has no right to it. This act falls under taking money without right, which is forbidden and is called "usurpation" (ghasb) in the Arabic language, branching into types such as betrayal and deception.

If the father left an inheritance, then the questioner must repay the granting entity from it what his father took unjustly, or deposit it in public welfare. However, if the father did not leave an inheritance, then the questioner is not obligated to repay this money, although it is commendable for him to do so to fulfill the right on behalf of his deceased father.