Fraud by Land seller

Hello Sir, We opt to buy a land 4 years ago through a seller. At that time price of land was around INR 3 Lacs which we paid in some 12 installments. For some of the installments we were provided with an acknowledgement slip whereas few were paid directly in hand. We could not make proper due diligence of seller at that time because it was through some known referral. After paying the full amount almost 2 years back, registry / agreement documents were signed wherein we paid the registry charge of INR 50,000. We were assured that formal signed agreement duly attested by competent authority will be provided within 90 days after No objection check. Later on we got a news that seller who claimed to be the owner of land was actually not the correct owner and there was some dispute case going on with the original owner. Since last 1 year, we have been following with the seller to either provide a registry document/ return our money but they always keep on asking for some grace period. From local people, we have come to know that there are several land dispute cases filed on the seller and nothing happens to them.. Please guide how can we recover our money back.. Yours sincerely