For Detroit, you trade for Cozens because you think he'll bounce back to being a young 30 goal, 60 point center that actually could be acquired in a trade because he's struggling.
For Buffalo you move him at less than his value because the team desperately needs a shakeup and you're concerned that Cozens is never bouncing back (at least in Buffalo) and need to recover assets before his contract is immovable.
Either way, Buffalo isn't getting 30 goal scorer value and Detroit (or whoever ultimately makes the deal) isn't getting the 15 goal, $7 mil cap dump price. If Buffalo moves him, they have to accept less than his potential worth. If someone else wants him, they have to accept paying more than his current worth.
Where his value ultimately shakes out is hard to say and I have a hard time imagining the GMs arriving at a happy middle ground. I cannot imagine HFboards arriving at that middle ground. Buffalo fans want 30 goal scorer value. Other fans want to poach for nothing.
Put another way, if someone from the future posted the real trade that is yet to happen, I think at least one, and probably both fan bases hate it.