A number of the comments on this and other threads reference Marner's no movement clause, and some of the side conversations reference Tavares' as well, and how they are now like a millstone around the neck for the Leafs cap situation.
It seems to me that these seem to be pretty prevalent in the NHL. I could be wrong (since when I was kid I never followed the nuances of contract details for players) but it seems like more of a recent phenomenon to include NMC in player contracts.
I understand to a small degree why teams do this, to incentivize players to sign the contracts, but in Marner's case, it seemed like a roll of the dice since I'm not sure if it could be shown that at contract signing, that he performed to potential. Maybe it was not so much about awarding him a bonus for performance; rather they would be worried that other teams may poach him and decided to make the first strike at retention. Still, seeing how the Marner and Tavares contracts have that clause...and I think Nylander's and Matthews does as well - it seems the NMC has a much more long term detrimental impact on the team than on the player.
It just grates me the amount of control and power the player has over their employers. Sure, teachers and professors can get tenure and Canadian senators have it too so they can't get fired, but typically that is after a certain period of service.
To give a player a NMC and then ask them to lift it for the benefit of the team seems like a backwards way of doing things (at least to me). In Marner's case, I would suspect if he can be agreeable to be moved, it would be on his terms and likely the return would not be to the Leafs' advantage. I think given his level of performance and what seems to be a rather unlikeable personality, the Leafs may want to just suck it up and take the loss (financially) on this one.