It's a very gray area, and I'm sure you'll hear a lot of varying opinions- it's been discussed before to death here. Just do a search for "Matsui".
IMO, unless your league rules explicitly have a case for this, you're stuck. I'd ask the other owner if he'd negate the trade (esp. assuming you've got some 2-day window to allow vetos). Or, you could ask another owner to call for veto.
As for Harden, I'd be hard-pressed to accept a trade-back. He's a known injury risk, and he's (hopefully) not out for long.