Validity of Unregistered agreement of sale

I have taken an amount of 300000 from a person Mr A and he took signatures on a bunch of blank stamp papers and took the original sale deed of my flat on July 2011. Later in the year 2012 July I told Mr A that I wanted to sell the flat and clear his debt and take the rest of the amount. He initially agreed and when I showed the original sale deed papers to Mr B they agreed and paid the amount of 800000 to me and when I wanted to return the amount that I owed to Mr A he said that he wanted a goodwill along with interest of 4 percent per month. I agreed to pay the interest but he wanted goodwill of double the amount he gave to which I did not agree and the matter was kept pending till 2015 and after others intervened he agreed to return the original sale deed papers if I paid interest till 2015. I disputed about the delay from his side and the matter was postponed due to my bad financial condition and now the Mr A wants me to give all my agricultural land valued at 1 crore or he will file cheating case against me for selling the flat to someone else. I has been 8 years since the signing of blank stamp papers and in the year 2015 he took blank stamp paper from my fried who stood as witness to this loan. The flat was in my mom name. Mr A now says he will file cases against me, my friend and my mom. We are willing to return the amount we have taken from him along with fair interest but Mr A wants all my assets. He has even taken my educational certificates and is not returning them. What are the cases he can file and what is the recourse that I can take against his threats