Disputed property sold to me which i came to know later ?

Dear Sir, In 2013 a local builder sold two flats (1BHK Each ) @20 Lacs ,me a property whose docs pertaining to chain of titles are incomplete.Later, i paid the whole money and in lieu of one flat he has purchased one flat from me in which we were resided.Our flat was sold to the builder @10 Lacs whereas the price prevailed was 15 Lacs but i was not having any option i sold my old flat to him @10 Lacs. Eventually, i was able to get the other flat financed and later paid 10,35,000+10,0000(Token money),,, at the time of registration and the builder agreed to register my house with GDA. Now, i came to know that the first owner's docs of the property is not produced nor found in Tehsil and he told that you have to pay extra 80,000 as it takes some time to get it financed by bank. Today ,after 3 years i got a call from the builder to pay 80,000 as it was not paid. I asked him to provide the missed docs but he abruptly refused and being a lawyer in high court threatened me to use illegal practices to get the money recovered from me.So, kindly suggest what should i do as i have been deceited by the builder as the title of the property is disputed. Anshuman 8800333899