New Dresden Files podcast interview.

[vincentric]

That depends on point of view, doesn't it?  So from Harry's stand point, she was moving the goal posts, from her point of view, merely making deals.

From the time of the second favor until Changes, Harry has no interactions with Mab. Becoming the Winter Knight, makes Harry her lifelong vassal and she no longer has to ask for favors because she can give orders.

We've had four books with Harry as Winter Knight and only in the interlude between CD and SG did Mab do anything that restricted Harry's actions outside of his assigned duties. Being the Winter Knight helps Harry and as long as Mab uses him to defend humanity, he won't need to lose the Mantle.

[morriswalters]

She said she "purchased" his obligations to Lea.  Then Harry struck a bargain with Mab to do three favors.  And yes, she does move the goal posts, whenever she has appeared and asked something of Harry, he thinks he is holding up his end of the bargain.  For the first two it works, but he never can
quite finish off the last one, Mab keeps opening loop holes to keep him on the hook until eventually he becomes her Knight.

He pays number two in Small Favor. He next sees her when he breaks his back in Changes.  She must be using magic to move those posts.

[Bad Alias]

I still don't see any goal post moving. When did Mab change the rules on Harry? If Harry had completed a third request, Mab hadn't replaced her Knight, and let Harry know she still wanted him for the job, would Changes be any different? It wouldn't.