Is it permissible for a son to claim ownership of land designated for him by his father before his death via an unregistered waiver, while the heirs refuse this and demand its division according to Sharia?

If the son did not fully take possession of the gift during the father's lifetime, then the gift is invalid and is considered an inheritance to be divided among all heirs, based on the statement of Ibn Abi Zayd al-Maliki: "A gift, charity, or endowment is not complete except by taking possession." This is also because Abu Bakr (may Allah be pleased with him) gifted Aisha a gift, but she did not take possession of it until his death, whereupon he said: "Indeed, today it is the property of an heir." Registration in the land registry is not a condition for the validity of a gift. Equality among children in gifting is obligatory; therefore, gifting to some of them to the exclusion of others without a legitimate reason is void. Hence, the son must return the gift for it to be divided with the rest of the inheritance if his siblings do not consent to him keeping it exclusively.