Is it permissible for the donor to revoke his gift that was not registered in the Real Estate الشهر العقاري [Registry], and he has a waiver document in the possession of the heirs, and what is the required action?

The grantor enabled his brother to dispose of the properties through the waiver document, and this is considered constructive possession. Consequently, the heir has acquired the gift, and the brother has no right to reclaim it, because the gift has been completed and it is not permissible to revoke it, as the Messenger of Allah, peace and blessings be upon him, said: "The one who revokes his gift is like a dog that vomits and then returns to its vomit." However, it is good to give him something from the inheritance out of consideration for kinship ties and his deteriorating condition, provided that every adult, sane heir agrees.