wow. you really can't even consider vetoing this one. bourn isn't really a good player. he might continue to get SBs for now, but he isn't even at all guaranteed to keep his job.
as a commish, you really need to look at the deal in the most favorable light possible without being unreasonable. don't worry about collusion - commishes who use that as the standard won't be able to veto Arod for Broussard without proof. just take the "losing" side and use the most optimistic reasonable projection and use the most pessimistic reasonable projection for the "winning" side.
broussard has shown he is capable of hitting 20+ hrs and batting near .290 (look at his stats 2 years ago) and he's off to a good start this year. if he becomes a full time player, he could hit 25+ hrs, with decent R and RBI totals. bourn has shown that he is not very likely to be able to hit for a decent average, and has a lousy one so far this year. ZIPS projects him at .250, and CHONE at .259. It certainly would be reasonable to think he could hit worse than that, as his bad current average might suggest. He has no power, and will contribute virtually nothing in HR and RBI. He could easily be moved down to the 7th or 8th spot in the order (if the Astros decide to move Pence back up), and he won't contribute much in runs either. he'll definitely steal bases when he is in the lineup, but most projections have him in the low 20s to mid 30s. that isn't really worth a tremendous amount.
this is the way every commish or trade approval committee should approach every trade.
you can downgrade guys like ryan braun, and you can look at guys like crede optimistically, but even if you do so, you those two are not going to look close enough to approve a straight up deal. but when you are dealing with 2 low end guys like bourn and broussard, you can't let your own opinions take over other owners reasonable projections.
i'm the guy that puts the "nip" in omnipotent...