First thing, I'm not saying I want Soulfire without some form of debt-balancing; that would be sick and wrong.  I'm just saying that the general Faustian bargain behind most sponsored magic seems inappropriate for Soulfire.  A sponsor who's all about protecting Free Will^TM shouldn't be subverting it through the dispensation of power.

Without getting into spoilers, I was thinking of the way things worked in "The Warrior" as a better alternative.  Allow a Soulfire-endowed character to work off their debt or even build "credit" by taking self-compels to intervene in situations for the better.  The table could adjudicate whether an intervention was properly motivated or simply a naked grab for sponsor debt.  Of course, characters whose concept or aspects lean towards them being an Arm of the Lord could take their compels in the form of divine missions, like the Knights.

Or has anybody come up with other alternatives that work better for them?