The ability to veto a trade is available in order to prevent collusion in a league. There are two ways to veto a trade, and in League Manager leagues, this setting can be changed by the League Manager at any time during the season:
Every team manager in your league has the power to cast a vote against a trade once one is accepted if they feel that it is unfair. As a team manager in the league you'll be sent an email with detailed instructions on how to cast a vote - follow those step and you'll be all set. If enough votes are cast against the trade (4 out of 10) then it will be immediately canceled. Teams with more than one team manager will only receive one vote
In this scenario your League Manager has the ultimate power to veto a trade or let it go through. The individual team managers still have a voice however. They can submit their comments privately to the League Manager where the other team managers can't see them. Only Your League Manager votes
If the rare scenario occurs where the setting is changed while trade is pending approval, that trade will be grandfathered in to use the original setting that was in place when the trade was accepted.