Is it permissible for a father to gift his house to his five-year-old grandson through a legally binding contract attested by witnesses, without officially registering it due to cost, and to write a will bequeathing ownership of the house to the grandson after death? And would this be considered a violation of Islamic law or a harm to the heirs?

Justice must be observed among children in giving gifts, but not among grandchildren or other relatives. It is permissible for a grandfather to single out some of his grandchildren with a gift or a bequest, provided that this is not a stratagem to increase his son's share. If a grandfather bequeaths a house to a grandchild after his death, the bequest is not enforceable except within the limit of one-third of his estate, and anything exceeding that goes to the heirs. It is recommended that a bequest not be made unless a man leaves sufficient wealth for his heirs, and it should not exceed one-third.