There's probably no union involvement because there's no attempt from the team to void the contract outright. It appears they will follow options spelled out in the contract and the league's labor agreement:
A trade would be the easiest way out, if there are any takers. The Stars probably won't cut him because they would owe him $4 million for each of the next three seasons. They're more likely to hold his rights until July 1, when they could buy him out for two-thirds of the balance ($8 million, instead of $12 million).
This just smells of the Stars having decided that they made a contractual error, and they're trying to correct it. I hope they have to eat the contract.