Added an extra player in a trade.
need a league manager to correct.
It takes you three steps to make an offer in trade.
Two of them after you put trade offer together.
If other manager accepts, the deal is done. Nothing a league manager or Yahoo can do or will do.
Live and learn.
Only thing left for you to do is send email to everyone in your league and ask that they vetoe it but they will only do so if trade is really stupid and lopsided.
This sounds like it.
Send the email to managers and hope for the best.