Forged sale deed

Sir/madam i purchased a open plot from a person 5 years ago constructed a small room got electricity connection i gave it on rent small minimal rent with out any rental agreement .recently when i want to sell property the party who want to buy got through ec found that property already registered 20 years back to other person(i dont know about ec before) .when asked the seller of my plot he told that the document first sold is forged .the way it forged was( as told by seller)on the front side of document sale deed all pages forged taking advantage of his illetracy the forged person got his signature and thumb impression on second side of pages with other plots documents registered genuinly on that day he kept this document in that bunch of docs .do the forged document is valid NOW THE PLOT BECAME DOUBLE IN COST .i approached forged document party(who took plot 20 yersback) he says he too got cheated give me some money to get the issue settled if i agrees for settlement it will be lose to me how to get forged document cancled