Does the grandfather's waiver of his share in his deceased son's inheritance to his grandchildren nullify their right to the mandatory bequest (al-wasiyyah al-wajibah)? And is it permissible for the paternal uncles to retract the division of the inheritance after agreeing to it?

A grandfather's gift to his grandchildren is valid if he is of sound mind, and he is not obligated to be fair between them and his own children. If the grandfather dies and leaves behind living sons and a deceased son, the inheritance is divided among the living sons, and the children of the deceased son receive nothing, as they are excluded by consensus (ijma'). The law of mandatory bequest (al-wasiyyah al-wajiba) contradicts Islamic law. It is permissible for paternal uncles to gift the grandchildren their deceased father's share, provided the donor is adult and of sound mind. Once the gift is received, it cannot be revoked, except for a parent in what he gives to his child. However, if the gift was based on the invalid law of mandatory bequest, then the uncles may revoke it.