Harry doesn't seem to be forced to do much of anything to 'pay' for his use of Soulfire; he just is weakened for a while (which translates here to taking longer to build up more Fate).
You could argue that Harry normally tries to do the right thing, even when he knows it will get him into trouble. It may just be that the sponsor isn't having to compel Harry to do anything he wouldn't normally be compelled to do. It just means that for some of those difficulties that come his way, there's no fate point reward because he's already had the points earlier on.
