Marleau's cap hit is 1 season. 1. Saving cap space for expiring ELC's is irrelevant, because they all have the room for next season. If anything, this helps them save that cap space for next summer when they may need to give raises.
2. And it goes without saying that we'd be giving up pick(s) and/or prospect(s) to create added incentive.
I'm just not sure I understand what the point is you're trying to say honestly.
Marleau would be low cost to the Sharks in the scenario he ends up there. Because after he is bought out by a third team, they sign him to the 1.2M or whatever that he is supposed to make this season.
If you're point is "what if the Sharks aren't interested" then of course none of this happens. But we are discussing this based on the assumption that Doug Wilson would bring Marleau back for a final season (and given they wanted to keep Marleau 2 years ago, and they keep bringing Thornton back and they are generally a loyal organization, it's a reasonable assumption) and that Marleau would be okay with facilitating a return to SJS (and given he still owns a home there, and says it will always be his home, is another reasonable assumption).