Is a notarized letter valid when only the person selling signed.

I sold a quarter of an acre to my neighbor some years ago and once the land was paid i found out it would cost us some money to subdivide he then decided to sell be back the land. I paid him the land and notarized a very vague letter stating that I was buying back the land and that he and his wife could stay in the property for as long as they desired. My neighbor passed away and his wife only has the letter notarized but claims I still need to pay her more money. My concern is does she have a case in which she own part of my property. The deed has always been under my name only and no other paper work has ever been made beside some receipts that I gave him when he was paying me and they only stated how much he had paid at that time.