Can we challenge illegal registration of our property by Registrar of Vijayawada by the outer peopl

This is regarding registration of our property done by Outer people who acted as our Builders in 2014.Tthey ran away without constructing the apartment. At that time an agreement registered between us & registered in Vijayawada registrar office.But the developers failed to complete building by showing silly reasons. The time period was completed by 2018.At the time of agreement in 2014 we as owners& Developers underwent an registered agreement according to which they failed to complete building& ranaway selling their share of flats. We waited& requested them to complete the building but they stood without any response to phone calls& to other ways of communication.After agreed time is over we with our own money completed the left overbuilding But after 7 years one of the developer registered one of the Flat of our apartment on his wife’s name claiming that he is having right to do that without our( landowner’s) permission. recently 20 days back. It’s illegal registration. We want to challenge the fraudulent registration done. More over he is torturing us by filing false cases in local police station also.Iam a lady of 58 years. I am being terrified with the acts of of Ex. builder trying to terrify with rowdies.Financially we are week. I want justice. Pls. help me how to stop that registration in registrar Office of Vijayawada. Can I seek free legal help from Government in this issue?