What is the ruling on a mother relinquishing part of her right to a piece of land to her grandson with her consent, and is it permissible for the son to benefit from it, or must it be returned to the heirs?

There is no sin on the mother for preferring her grandson in a gift (hiba), because favoring a grandson is permissible.

Regarding the grandson's ownership of what was gifted to him, there are two situations:

1. If the grandmother gifted the land to her grandson while she was in good health, and the grandson took possession of it before her death, then the gift is valid and becomes his property. If the land was not taken possession of until the grandmother died, the gift becomes void and reverts to an inheritance to be divided among the heirs, according to the majority of scholars. The Shafi'is and Hanbalis disagreed, holding that the heir takes the place of the donor in effecting possession.

2. If the grandmother gifted the land to her grandson during her death illness, it falls under the ruling of a bequest (wasiyya). It is valid for the grandson if he is not an heir, provided that it does not exceed one-third of the wealth, unless the heirs consent.

It is advisable to refer to the Sharia courts to settle these matters.