I think I explained it pretty clearly, you're just not getting it. Marner has no way to stop Toronto from trading their
negotiating rights, there's no movement involved, Marner can still say no to the new team with negotiating rights but they'd have exclusive access to talk to the player and make their best pitch for a deal before July 1 when he hits UFA status.
Here's a Google search that might make it clearer:
"In essence, trading for a player's negotiating rights is a strategic move that can provide teams with an advantage in the pursuit of talent, particularly before they become unrestricted free agents. "