Ha! Well, to give you a tiny bit of psychotherapy, as long as the favor is freely given, doing a favor for someone you care about really is a gift to both parties. The relationship is enhanced by it. Both of you become closer as a result. That's a hard thing to internalize, I know (I'm kind of like you in my aversion to being socially indebted) but it really is true. That truth, plus the traditional potlatch of pizza + beer, is enough.

I am about to hire an economist who's a very creative thinker. I'll discuss with him, I'm sure he'll have a bunch of ways to make this concrete. Will get back to you!