Well man, if I read that correctly, you put yourself in a pickle by putting it up to vote. It was 5-4 FOR veto, correct? So your choices are to let a theoretically fair trade be vetoed or let it stand and go against a league vote.
I would take the opportunity to go back to your owners to constitutionally establish veto rules. Just tell your guys" sorry, I got in a hurry, but this situation will come up again so let's decide the rules now." By opening that discussion, you may be lucky enough to find a solution to the current situation.
Otherwise, it was voted on and you have to veto it. Unless voted on veto meant NO veto