I’m a MN fan and have no interest in Domi, but purely on value MN obviously needs to add
I'm pretty low on Greenway, you can ask 2Pair, but if the idea is that he's a 35-40 point left wing with his size and puck protection skillset, I don't think he's that far removed from the 50 point left wing that Domi is. A 2nd rounder is probably about right, when you consider Domi is probably looking for a 6M contract, maybe more. Given that Firstov was a good 2nd round selection that progressed nicely in his D+1, the value is probably pretty close if Montreal were looking for a deal that could replace Domi now and also provide a nice future asset.
Looking at what a guy like Zucker got with 3 years left on his contract, a late 1st and Addison is pretty equivalent to Greenway+Firstov or 2nd. Kapanen got 15th oa and a bottom 6, grinder style left wing, and most agree that was a pretty severe overpayment. That's probably realistic to the type of return Domi would get in a package of futures deal.
In any case, I'd prefer to hang on to Firstov, as I really like his potential. Swap him for the '21 2nd, maybe you can add a '20 or '21 4th, call it a day.