Poolman my friend, I think in the back of your head you know what I'm going to say.
Would you go to buy a car, hand over the money - and then drive off without the car? Pardon my french but - FUCK THAT.
What you should do is simple. Sex is a fair trade, and if one person is taking the goods and not giving it out now that isn't fair.
STOP MAKING HER ORGASM.
Is the the simple answer. A little bit of making out is ok (give her a taste of what she's missing), but heavy making out and anything else you want to give her is strictly off limits - how hard it is.
Personally be fine - treat her friendly, with a bit of cocky funny, but DO NOT go further.
If she asks why you're not doing anything, pull some cocky funny and say that you aren't gonna give until she pays back her debts

and change the subject.
I'd say you have two possibilities of outcome; she'll cut her self loose or go for your cock. And I'd say that letter is more likely - especially if you keep it flirtacious, but frustrating on her part.
Good luck!