Trouba is very good, but reports are he wants to be paid like a #1 D-man, not a first pair D-man, which would be a sizeable overpay (he's not a top defenseman, and after six years, probably will never be).
Ghost has four cost controlled years left on his deal, is as good defensively and better offensively than Trouba, but his size limits his usage, will always struggle with the physical aspects of the game, so you have to shelter him to some extent. Trouba can play the big minutes.
If the contracts were equivalent, a Ghost for Trouba trade would be a good way to balance the pairs.
But the contracts aren't close to equal.