At first glance, even I read it like you said "exactly" to his last part about grabbing. HOWEVER, Aethon is not dumb. You clearly said that he will keep the things after the project was done and he knew this.
I think the punishment should not be for grabbing the crits, it should be for trying to play smart and use language twists against you, to try and save himself from trouble.
But the punishment should not be big, because it was indeed a partial misunderstanding. That is why I voted for: "Confiscate the goods aquired and do nothing more". I would also give a warning not to do it again. And definitely not trust him with this OP stuff in the future.
P.S: Mur, next time when giving such OP items, make sure everything is clarified lol
P.S. #2: If this was a first-time case, I would not punish Aethon. But he has a history of such stuff, unfortunately.