That means you understand why a debt's important magically, right? Someone pays their end of the bargain, you gotta pay yours, or shit gets dire. That's the core of a deal, when you get right down to it.

So...

[She spreads her hands, smiling benevolently- with maybe just a hint of mischief.]

Make it pay up.

You got your power by contract, but you paid for it same way as any seer does- with every last little bit of pain in the ass bullshit your power puts you through. Dealing with that crap is our sacrifice in exchange for getting to use this power, so you know you've delivered your part already. And sacrifice like that, payment like that... gives you the right to call on it, magically. It's got a kinda power to it.

You don't gotta strain and flail to pull out the power, it's yours by right. Just tell it to pay the fuck up already and wait, don't gotta get all stressed about it. You're not the one who's debt just came due here...

[She's not sure if this is going to help, honestly, but she figures that putting it in a more familiar framework for him can't hurt. If nothing else, the confidence boost might help him relax. To stop him from seeing his power as something foreign and scary and start seeing it as something understandable, that has rules...]