Must a father financially compensate his other children if he forgave one of them from paying part of the price of a car, and does the one who was forgiven owe a debt to his sisters for this amount?

If a father absolves his son from a debt owed to him, he is not permitted to revoke this absolution; because it is an act of relinquishment, not ownership transfer, and what has been relinquished does not return. If the absolution is in the sense of a gift, then the principle is that the father should act justly among his children, giving the rest of them the equivalent of what he absolved for one of them, unless there is a reason that necessitates specificity, such as the son's poverty, need, large family, engagement in knowledge, or his being a transgressor (fasiq), or an innovator (mubtadi').