Ezran being instrumental in either Callum's knowledge of what the cube does, or even in being able to use it because it may be linked to his blood line in a similar manner of Kim'Dael's mercy debt? Unless it's a subversion, maybe, (and a point of contention) that Callum can use it without him. Or, if Callum does outright trade it and it's not the boys working to save Rayla, it's gotta be for Ezran: "But as a father, I have a selfish wish: for you and Ezran to be free."