Validity of agreement document

There was an agreement of sale of our land with a buyer A. He paid an advance and agreed to pay the remaining in 3 months, documented and signed. But, the sale didn't happen for more than 3 years. We then found buyer B, returned d buyer A's advance n sold the property to buyer B. On getting his advance back, buyer A gave us back d advance agreement document, facilitating d sale. But, now after 5 yrs, from d original date of agreement, buyer A has come to us saying he wants the advance amount returned. When questioned, we found buyer A gave us a color copy of the original document to us instead of d real one and now is trying to cheat us out of money by claiming he has the original letter and that we did not return his money to him. In the document we received from him, buyer A has written, in his handwriting, that he has received the the advance amount and he acknowledges that there is nothing further of business between us. But, now he says, that letter is a forgery and he wants his money. It's been 5 yrs since we first dated that document and we had believed he returned the original document to us in good faith once he got his advance back. How do we proceed in this matter now? Kindly advise.