The claim that the equality of a trade depends entirely on the salaries involved is ridiculous. Equality of a trade depends on the value of the players traded, and salary does not track the value of the players perfectly, or well at all in some cases.
That approach would seem to imply that this is a fair trade:
1919 Babe Ruth: 641 PAs, 322/456/657, for a 1.114 OPS -- $8,478,968
for
1893 Duke Esper: 420 IP, .306 opponents average, 1.79 whip, 79 HR+ -- $8,519,255
Clearly, Esper's salary is high due to his innings, but they are terrible innings and terrible innings have very little value in a league at this cap. Ruth's massive OPS in a pitcher's year, on the other hand, has very much value in this league. Equalish salaries, unequal value, and such a trade clearly should be vetoed. A fair trade would be Esper for someone with significantly less value than Ruth. Such a player may well have a much lower salary than Esper (say, something like Mark Teixera's partial year with the Angels? good LHP half of a platoon at 1B for 3.2 million?).
So unequal salaried trades definitely can be fair. Whether this one is fair I don't know cuz I haven't looked at it.
You simply can't (sensibly) make your veto decisions based simply on salary.