Does he have grounds to file a law suit against me, as I feel that I have done nothing wrong and it was a as is trade?

I recently made a trade with someone online for my antique firearm for a sand rail. When I made the trade, the guy with the sand rail brought a gun expert to examine my firearm for authenticity and value and determined that it is what I said. It was and a fair trade. We made the trade and signed a paper saying it was as is. After the trade, he told me he disassembled the firearm to look for a signature which is not there because it wasn't signed in the first place which now has dropped its value because it was in 99.9% condition only fired once by factory and never disassembled. Now he has demanded that I give his sand rail back. He is saying that I falsified documents which I did not. He claims he used to be a police officer and is threatening to sue me if I do not give it back. I have put work into restoring the sand rail so I offered to give it back if he pays me $1000 for the work I put into the sand rail. He refused and threatened to sue me again.