Little help need

if you used steam trading system then i guess support can handle it.

I don't think a person can undo a trade. It was probably bought with a stolen credit card or something. And they revoked the game.
I guess you could contact steam "support". But I don't think steam support ever helped anybody before.

same trade window, steam support help.

not same trade window, you got scammed and no help. lession learnt.

He paid for the game with a stolen credit card or did a chargeback on the game. If you traded your keys for the game in one trade window you giving keys and taking gift copy of the game you can take them back, steam support will help you.

karma

report him on steamrep.com too