At some point, Mab gets cornered into giving up the "favors" Harry owes her to someone else - if you get into why or not is up to you. (Bonus if it's the Erlking, who wants to have the favors from the mortal who managed to summon him, but I'll also take Odin or Lily, or someone else even.) The one who now has the favors holds it over Harry's head, and teases him about how he might ask for the favors fulfilled.

Author's choice as to timing (how many favors Harry still owes, and how far along in the series they are).