This one's borderline, in my book, but right at this moment, I would let it through.
Argue for it. On Sporting News, Walker is ranked 23 in outfield while Sosa is ranked 9. That's not as great of a difference as I expected it to be.
Sosa's been in a slump. Walker is getting hot. Sosa's out for another 8 games starting next week. That's two weeks of production, which is definitely significant.
Overall, it's a fair trade, but you definitely win. Make the case and ask the commish to publicly argue for his veto instead of just forcing it down your throat.