Is there a way to trade cities?
Solution 1:
No there is no way to trade cities. I think in the Warhammer lore, no faction will trade their cities with another faction, even with confederations.
But, when you keep all cities of a region and you get the benefits, if a faction that's your vassal has a city on a region and you have the other city on the same region, you can gain the decree's benefits, but you will eventually confederate with that faction. There are huge benefits in having that city beyond just getting some money from your vassal and having the decree.