Were it me, I'd offer the 1911 back on the condition that I was given incontrovertible proof that the P220 was indeed stolen and in LE possession.
That sounds like the noble thing to do, but I don't think it would necessarily be wrong not to. The trade was fair and square. It's unfortunate, but equal risk is assumed. I know I'd want mine back but wouldn't hold an established member to it. A new member, however, might be a different story.
Don't do anything until the authorities come show you that the gun YOU traded was stolen. There's nothing to show that the guy didn't already have a stolen gun and is setting you up for a loss. Make everyone prove everything first and I mean without any doubt.