Man, this is a funny story. Lol!
Believe it or not, I've seen stuff like this happen in leagues before and I've even seen corporate level guys act this way and try to get each other fired over this kind of argument more than once.
The guy has no case though. Not at all. The trade was vetoed and it never took place, so he has nothing to worry about. He wasn't cheated and even if he thought he was it was rectified. You were wrong to go into his account though and do what you did, but you got it fixed, so no harm no foul at that point.
It sounds like he just doesn't want to pay his dues on this and he's looking for a way out of it. The league commish shouldn't have to pay anything on his behalf. If he doesn't pay, then he should never be let in the league again.