Property already sold can we cancel the deal

My grandfather had sold ancestral waste land before 2 years with the consent of his wife, 2 sons & 1 daughter. But even after 2 he haven't received full sum of money & these all people who have initiated this deal are illiterate & they are cheated by the counter party. Counter party have taken power of attorney also. Now my grandfather wants cancel this deal. So kindly suggest how to move forward.