I'll throw this out there too. Harry owes Mab a favor, plus, he's the Winter Knight, which means he owes all of the Winter Queens unlimited favors, excepting only that he doesn't have to accept any that would hurt the ones he loves.
He also owes Molly a favor from bargaining for the ring in Peace Talks. She warned him to not make an open-ended deal with the fae, but Harry's is remarkably dim in foresight.
Harry owed Lea a debt, and somehow, Mab wound up owning that debt. It's possible she ordered Lea to surrender that debt to her. We don't know what went on behind the scenes. Mab could do the same with Harry's debt to Molly.
Harry's favor to Mab is a conditional, he doesn't have to accept the favor asked. But Molly's debt has no conditions attached to it.