Is it permissible for a person to bequeath one-third of his wealth to his grandchildren who do not inherit, and to وقف (endow) an orchard valued at 100,000 Saudi Riyals? And is the وقف (endowment) calculated from the one-third or is it in addition to it?

What the inheritor gifted to his grandchildren during his lifetime is considered their property, and it is permissible for it to exceed one-third.

As for the endowed orchard, if it was an accomplished endowment during the endower's lifetime, then it is valid, whether for the heirs or others. The important thing is that the endowment is not contingent upon his death, because then it would fall under the category of a will. In that case, a will to an heir is not valid, while a will to a non-heir is enforceable within the limits of one-third.