this is a really tough situation. It's clearly not collusion, but clearly a bad trade that the guy is probably starting to regret. However, in my opinion you can't veto this trade. To the guy who just offered more- he should have offered that beforehand. There is one league I'm in which is all friends, and we all chat outside of the board. There have been MANY trades in that (keeper) league (especially between two guys) where I wanted to veto and offer better. But the fact is, if an owner doesn't get full value because he doesn't shop around a bit, that's his own fault, and good luck for the other owner.
I think this is a perfect example of a very lopsided trade that can't be vetoed because there is no collusion.
That said, if your league has decided there will be a review of trades, and lopsided trades without collusion have been vetoed before, then you must veto. But then I think you have your vetos set up wrong. Good luck!