There is also another "trick" that can be implemented but it's a little harder now with "cool down" rules and that is where you have a faux war and allow your ally to take those lands from you and then come back to the coalition after the fact.
In addition to trading province for province , you can sell a province to an allie... so you have trade and sell to deal with this.
I am not positive on exact limit of selling province, but for trading it is +/- four per map, I will do some testing to determine if the sell province is encompassed in the +/- 4 per map game mechanic
4 Provinces in deficit, So it mean you can sell 12 provinces if you gained 8. (12-8 = 4)
One annoyance though in province for province trade if you're at a deficit of 4 it demand you to first get a province then give a province. The mechanic isn't smart enough to determine this is a province for province trad