What is the ruling of Islamic law regarding a son taking possession of a house that he had gifted to his father after the father's death, knowing that the contract was written as a gift for the costs of transferring ownership, and that the amount was paid in the presence of witnesses?

For a sale to be legally valid according to Sharia, legal documentation is not a prerequisite. The brother who sold the house and received its price must fear Allah and not seize it. His other brother should advise him, and if he insists [on his stance], the matter should be referred to the Sharia or civil court, with the help of witnesses to prove the sale. The fact that the man carried out the procedures on behalf of his father does not make him responsible for depriving the heirs of their rights; rather, he must strive to remove the injustice from them. Even if the house was gifted, the donee owns it by virtue of possession, and the donor has no right to reclaim it.