Well... some team could offer him anywhere from $8,454,872 - $10,568,589 and only give up 2 1sts, a 2nd, and a 3rd rounder.
It hurts the Leafs because they'll likely match, putting them in a worse cap situation.
Best case for the offer sheet team: Leafs don't match and they get Mitch Marner for a smaller package.
Worst case for the offer sheet team: Leafs match and it messes up their cap situation even more.
Although we never see offer sheets, and won't this offseason.
Just for fun just the Leafs lose Marner for $8,454,872 - $10,568,589 and gain the 2 1sts, 2nd, and 3rd rounder.
They then have enough cap space to try and sign Panarin, and dangle a couple of 1st rounders to another team for a top 4 dman.