Unauthorized possession

My father and mother has purchsaed a agri land in our village at bihar from their PF funds on April 2019 and on August 2019 we have qpplit for e mutation.. On October month we came to know that one of relative(nephew) of seller has submitted a application stating that possession of sold land is in there hand and they were cultivating this land from last 10 years so CIRCLE officer of block has cancelled our e mutation. Now CO is very adamant about his decisions as relatives of seller has local contacts in block. We want to clear that they don't have any documentary evidence of land nor seller has authorized their relatives to cultivate in landalso seller has given a written application that he has soldbhis land without any pressure and good state of mind and also that he has never authorized any person to cultivate. My mother she is alone and my father being called defence personal is not in his home town .. We have lodged a complaint in local police start also but they are saying since mutation is not in your name we can't do anything. Please let us know how to settle this illegal and unauthorized possession of land. Prawin jha