Agricultural Land sale of agreement

I have purchased a Agri land. Total cost of the land is 95 lakh and paid a advance of 28 Lakhs and made a sale of agreement over 500 rs bond paper.(Its unregistered) we have written in the agreement that remaining amount will be paid within 6 months and get registered. Land was in Mother and son name. (It was ancestors property i.e father had bought it and came to mother name when father died. Seller family tree --> Mother,son and three sisters (muslim family) Mother and son got it transferred to their name when their father died by giving wrong information that they don't have any sisters. Now, one of sister among three have gone to court asking for her part and she has got stay from court to stop selling to others. Court stay has come within 6 months time period what we have in our sale agreement. I could not registered as there is stay from court. Now, seller is denying to pay me the amount (28 lakh) what I had paid him nor he is trying to solve his sister case. Its been 2 years from the date of our sale agreement. I did not send a legal notice to him within 6 months of our sale agreement saying I am ready for registration. How to get back my money now? Does my agreement holds valid if I go to court now?