Hindi car sale agreement

Hi I have sold my old car to one party. We have already signed form 29 and given to party in February 2019, but till date they have not got changed the registration of car. Everytime I am following up with them, but they are saying that we have sold this car to another person and he has applied for registration of change in owner name. Shall I give a general complain to police station regarding this or should I prepare a sale agreement and get it signed and get it notarized. If yes, please share a sale agreement of car in Hindi or Gujarati on my mail id mahhernosh@gmail.com, Please let me know what to do, as I have sold this car with my mechanic person with whom I was gettting my car service.