Is the grandfather entitled to sell land that he gifted to his son and then needed, and is the son permitted to reclaim it by paying its price to the buyer?

If the land has not changed and no right has become attached to it for anyone other than your father, then your grandfather has the right to reclaim it from him, based on the hadith (definition): "It is not permissible for a man to give a gift and then take it back, except for a father concerning what he gives to his child." As for the father returning the land to the one who bought it from the grandfather after his lawful retraction of his gift, there is no harm in that if it is by mutual consent and a valid sale. Otherwise, the owner has the right to refuse to sell his property.