Maybe because I'm a Commish or co-Commish across a few different leagues I have a different POV. But my largest priority in every league, even those in which I'm just a plain old vanilla owner with no responsibilities whatsoever beyond my own roster, is upholding league integrity.
Why? Because I love this hobby, and I really enjoy my leagues. I've put a huge amount of time into my leagues. Too much, maybe. But that's probably true for anyone in the Shark Pool in July. I've put too much time into drafting, bidding, looking over rosters, trading, emailing about trades, chatting in the league chat, pouring over possible starting lineups, getting jittery on Sundays as the clock ticks down. Too much time and investment to risk a league folding, or eventually folding, over a moment where I decided to take clear, egregious advantage of a complete and utter moron. My leagues staying in existence and staying enjoyable, justifying the time I've spent in them, are of utmost importance to me, especially since I share many dynasty leagues with a bunch of the same owners.
I reject this offer and send back a note saying that I can't take that deal in good conscience, and that Green is worth a TON more. I tell him I think stealing Green for that wold unbalance the league in an unfair way, and hope that it sinks in rather than making him take a similarly bad offer to someone else.