Can they mutually agree to terminate the contract? If so, then Trouba could sign with the Islanders or Devils for whatever they can afford. He's already made $50 million and could probably make another $20 million even if he forfeits the $12 million he has left on this contract.
I believe they could, but the Devils are already basically capped out and set at RD with Hamilton, Pesce & Nemec. Not sure on the Islanders situation.
Realistically, with $12M remaining on his contract, Trouba might only recapture a third of that value over the next two years if he went that route. Islanders would hold all the leverage on any contract for him this season.
I don't see any way he could still make $20M the rest of his career. For example, The Athletic values Trouba at $2.2M now and his value will only go down as he ages.
Trouba and his agent would have to believe that there was serious interest in a team not on his no trade list in acquiring him (likely with the Rangers adding enough incentive via retention and/or attaching assets) or some team picking him up off waivers at his full contract (which I personally find doubtful).
I personally still think he reports to another team if he gets traded or claimed off waivers as that is a lot of money to give up even considering his career earnings. He understandably is just doing everything within his power to prevent that from happening.