Is it permissible for the younger brother to retract his waiver of his share of the inheritance to his deceased brother and claim it for his heirs, due to the failure to complete the legal procedures for the waiver?

A gift becomes effective if its conditions are met: that the giver is qualified to dispose of it, and that the donee takes possession of it before the giver's death or bankruptcy. In this case, the giver does not have the right to revoke it. However, if the gift does not fulfill its conditions, it returns to its owner. This is based on the Prophet, peace and blessings be upon him, who said: "We are not like the evil example; the one who revokes his gift is like a dog that returns to its vomit." Also, Abu Bakr Al-Siddiq revoked his gift to his daughter Aisha because she had not taken possession of it.