Disclose Contract to Third Party Neighbor - Bad Idea?

[Luke Stone]

I'm debating if I'm crazy for considering this. The owner of the second half of a duplex I recently purchased has asked me if would provide him with a letter stating how much I paid for my unit so he can appeal the property taxes. The recent tax assessment came in $20k higher than what I purchased my unit for in May. So I am considering doing the letter or even just giving him the contract minus some personal info. If he wins his appeal it will help me in mine. His appeal hearing date is way sooner than mine... Also I would like to eventually buy his unit and I got a really good deal on mine (seller was distressed and needed to sell quickly). I think this may set a good base price for negotiations. Are there any major pitfalls to disclosing a contract?