Yeah, this is one of the trickier scenarios in HBD. You could try to get creative by asking the other owner if he's willing to either a) make an offer based on the assumption that he will re-sign and then reverse the trade if he ends up not wanting to re-sign, or b) make a trade assuming he won't sign with a promise to include other pieces if he does. Okaying it with the commish and world beforehand is probably a good idea.
Then again, that might just be more effort to coordinate/convince then it's worth but if he's a true stud...