How to cancel registered gpa agreement with builder?

My mother had entered into a registered joint development agreement with a builder for construction of residential apartment building on her land at a 50:50 share in 2011 . there was a previous court case pending at the same time and the builder was made aware of the same and he agreed to solve the case and complete construction of the apartment building. The same builder had cunningly mentioned a secret clause ( without our knowing) in the agreement stating that incase of any delay in construction, there would only be interest given at a nominal rate of " Rs 1!', instead of Rs 10!, which was previously mentioned to us before signing the agreement. (Maybe the builder changed one of the paper in the agreement in seconds before signing of the agreement?). Prior to this agreement, my mother had a pending court case with a backtracking buyer who made my mother sign a sale agreement and this case was recently settled by the current builder by him paying compensation of 10 lakh in court and rest of the cash outside court in 2014 february. Even now, Till date , this builder is not constructing or developing our plot of land till date and demands for the original property deeds to be handed over to him inorder to construct the apartment. Is it really necessary to handover our original property documents to the builder ? As we do not trust him anymore and would like to cancel this registered gpa joint development agreement now. The builder is now demanding 50 lakhs compensation from us inorder to cancel the agreement or let him construct the apartment with a new registered agreement again! The builder is harassing by careless replies and rude attitude . my mother who is now very weak and old. We dont even have a house to live. its been almost 5 years since the agreement was made with this builder whereas he agreed to build the apartment building in 18 months time.!