Doesn't it mention getting "temporary access" to powers/spells in the Sponsored Magic section? Then you use sponsored debt to represent the pact....*flipping through pages*
Edit: here it is on p289 the little yellow box:
Temporary Access
Sometimes what you’re looking for is a
temporary fling with power, rather than a
committed relationship. If the sponsor is
willing, and you have the ability to negotiate
with it in some way (whether by ritual
or through an empowered representative),
you may gain use of that sponsored power
and all of its benefits for a single spell. All it
takes is the acquisition of one point of debt
per spell. Though sometimes, the first one’s
free…
Our best example of this is in the
Proven Guilty casefile, when Harry was
able to draw on the power of Summer briefly
by using a mote of Summer flame that the
Summer Lady, Lily, had sent along with him
as he investigated Arctis Tor. This is also a
common method used with places of power
(page 292), where the spellcaster temporarily
draws on the ambient power of the location
without making a deeper, long-term binding
effort. In all such cases, the use of the spell
has to fit the agenda of the source—the
opportunity just isn’t there otherwise
Basically, you make a bargain with an entity and you pay Debt to use the powers. You make it up to the entity at some other point(via compels). You could do this by making bargains with multiple entities and have multiple debt owing out. Seems VERY risky...but FUN!
Not sure how to model it. Maybe that's what sinker and mstorer and suggesting. Personally, I like the idea of not having to pay the refresh for the debt part. You'd have to pay refresh to access the Entities through summoning/binding/cohersion or by whatever other means you have (Ritual is the first thing that comes to mind, but there could be other avenues - worldwalking perhaps)...everything else: haggling a deal with a creature can just be done as some kind of contest (social/mental) and then, if successful, you have access to whatever powers. Or maybe the character dies an Aweful, Horrible Death.