Buying out his contract at the end of the season will leave the Rangers with $883k of dead space for the following two seasons, which is very manageable.
While I do think that the Rangers would probably be willing to retain some salary, it probably doesn't make much sense to be retaining more than 30%. Equally, if they go the route of getting an ugly contract then it can't extend beyond this season as just buying ADA out at the end of the season would be better than an ugly contract on the books for 21/22.
It's not like they are completely over a barrel here.