Cousin sold my fathers agricultural land to 3rd party, how to get back

Sir, my father purchased farming land since 30 year back. He had only made aggrement on 5rs stamp paper as we had family relationship with them but now his son sold it to third person. Still now we are faming on that land from 30 years and all villagers are in our side. Can u help us what to do in this case ,where to go to police aur tehsildar or somewhere