From my understanding, the Leafs would have 7 days to decide to match or not. During that time, the other team and the Leafs can negotiate a trade. If a trade is agreed upon, then the other team gets Marner at the OS parameters.
I could be wrong, but I don't think I am. I remember all of this talk during the Shea Weber OS fiasco with Philly.
10.3 Offer Sheet and First Refusal Procedures.
(a) When a Restricted Free Agent receives an offer to sign an SPC from any Club
(the "New Club") other than his Prior Club, which offer the Player desires to accept, he shall
give to the Prior Club, in accordance with Exhibit 3 hereto, a completed certificate substantially
in the form of Exhibit 6 attached hereto (the "Offer Sheet"), signed by the Restricted Free Agent
and the New Club, which shall contain the "Principal Terms" (as defined below) as well as all
other terms of compensation of the New Club's offer. The Prior Club, within seven (7) days after
the date it receives the Offer Sheet, may exercise or not exercise its Right of First Refusal, which
shall have the legal consequence set forth below.
Once an Offer Sheet for a Restricted Free
Agent has been received by the Prior Club, the Prior Club may not Trade or otherwise Assign its
Right of First Refusal for such Restricted Free Agent.
Though it was pointed out to you before... but you disregarded. Here's the full explanation. You cannot trade the player.