I think there is an outside chance the Isles can keep Mayfield, mostly if he takes another discount. He'll most likely get at least $5M on the open market. If he stays for, say, $4.5M I think they can afford to keep him. While sure that's not what you want to be paying guys to play on your 3rd pairing, I think the fact that their top pairing guys are only making $6.15M and $5.75M should allow them to spend a little more on their #5. They don't have that $9-10M defender eating up a big portion of their cap.
They do have Varlamov coming off the books. If they are able to include a guy like Bailey in whatever deal they find to bring in a big name winger, it should put them in a good position to re-sign Mayfield - even if they do manage to keep their big trade acquisition(assuming there is one).
But yeah, unless the Isles collapse, I just don't see them dealing Mayfield even if they expect to lose him at the end of the year.