Next year (because you should not change rules mid-season) you should do a league wide vote. Then, when members disagree/veto a trade, they have to post WHY they do. Then the two managers proposing the deal can argue their side and you can have a discussion about the validity of the trade. You can throw out stats, numbers etc. to back up your trade.
Someone could easily argue that Halladay is on the downside of his illustrious career. Seems as though your commish (and some leaguemates) are caught up in his last start which was spectacular. Is he an ace? Yeah, he still is. But his K numbers are not outstanding and TOR is not a good team so W's will be hard to come by (off the top of my head, I would guess he doesnt get astounding run support). Now Bruce could turn into the next Ryan Braun. Or he could have this fiery start and cool down during the stretch run. Thats why you make these trades. It is based all upon potential. If you measure the potential of all the players involved (with pure speculation) then I think it is a fairly even trade. Of course you cannot compare the proven track record of Roy Halladay to Jay Bruce. Look at Bruce's sample size! Your commish is trying to compare apples to oranges in comparing a veteran to a rook. I wonder what he was thinking last year with Ryan Braun....Since he was drafted low he surely couldnt be valuable right? Just because a guy is taken with a late round selection does not mean he cannot produce like a top selection. See: Nate McLouth, Joe Saunders, Shawn Marcum...the list goes on. And many think Bruce will be included in that category. Look at his home ballpark. Look at his Triple A numbers. Look at the lineup around him. Like any player he has things working against him as well (Dusty and his K numbers in Triple A).
So, if your still reading this and havent moved on to the next post, I suggest you change the trading rules for next season. Write them down and have a hard copy that is distributed to league members.
Good luck the rest of the way.