I don't think this would count anyways because you took the risk of trading and some people are mean like that and steal. Next time, force them to give it to you first and then give them your half of the trade.
Plus you would have to have the full player name not just jack. I think I know who you're talking about though, but I don't know the player name.
Use a chest next time, mods SHOULD have a thing calls logblock, where it lets them see who did what on a block.
No evidence was given to us. So I cannot deal with this, sorry. Locking and Archiving due to inactivity and lack of evidence.
