Should the two grandsons, to whom the grandfather bequeathed a share, be given their share upon the sale of the first plot of land, or is it permissible to delay that until the rest of the inheritance is sold, due to their brothers' need for money?

What a father gives to his son during his lifetime, and the son takes possession of it before the father's death, is a valid gift. As for what a father bequeaths to one of his heirs after his death, it is a will that is not valid unless the rest of the heirs agree to it, due to the hadith: "There is no will for an heir." If the heirs do not agree, then everything the father left behind is divided among them according to the legal shares. If one of the heirs requests division, his request must be accommodated.