What is the ruling of the Sharia on a grandfather's will that favors two of his grandsons over their siblings in inheritance, on the grounds that they worked the land and did not receive an education, while the others did?

A grandfather's bequest to some of his heirs, granting them more than their legal share, is considered a bequest to an heir and is not enforceable unless the remaining heirs agree. The service rendered by some children to their father on the land does not necessitate the enforcement of the bequest in their favor. Rather, they are entitled to receive wages commensurate with their work from the estate if they were not acting voluntarily, and if it was customary to receive wages for such work. However, if it was not customary, or if they were acting voluntarily, then they are only entitled to their legal share of the inheritance.