If you make the trade stand, is everyone in the league going to turn on you? If they really will think that much less of you, just put it up for a league vote and see what happens. I know we all love fantasy baseball (it's why we're on the Cafe), but it's not worth everyone getting pissed off at each other, especially if you guys are friends.
I'd put it up for a vote. If the majority decides it shouldn't stand, then go with that, even if "technically" the deal should stand like everyone's been saying. It's just fantasy baseball in the grand scheme of things, it's not worth it for some huge issue to come up.
"Oh, that Lankford and McGee, the trio of 'em. They're a one-man wrecking crew."