Transfer agreement of parking lot between previous owner of my flat and neighbor

Hi, we bought our flat in year 2005 from Previous Owner (P.O) and our neighbor(who is also a lawyer) is claiming that there is a agreement done between them (neighbour and P.O) in 2000. However, the P.O is not aware of any such agreement. there was some legal help provided for the previous owner by the neighbor (around 2004-5) and some signatures were taken by the neighbor during that time. However, during the sale, it was never mentioned to us about the agreement. We shifted to our flat only in 2011 and only recently we came to know about this swapping (there is no labelling of the parking slot). when we asked for the agreement, the neighbor took time to produce the agreement. From the FONT and the color print, i can say that agreement paper was recently printed out with the content, although the agreement is dated back to 2000 (catch here is the 2000 year looks like 2007 was somehow changed to 2000). How should we proceed legally to expose them for using fake agreement paper? can we provide a notice to them, requesting to submit the agreement to court and then get it validated with the forensics? please suggest