Let's look at this logically. I cannot, under any circumstances, let it go. Letting one trainer go, while taking money from others is just wrong.
Now, what I really want is her. Raping her would just get me in trouble, so I need to make sure she is willing. So, the best way to go about it to just challenge her again. She could win this time, and it could make it even. Obviously, though, that is not going to happen. This doubles her debt. So far, so good.
At this point, I can try challenging her again. Give myself a handicap of some sort and tell her that I'll forget the debt completely with just one loss, to give her hope and a reason to believe she MIGHT win. If this works, good. If this doesn't, ask for her number and tell her I really enjoyed the fight and that I don't care about the money as long as I get to fight her periodically.
Once it's gotten to the point where I can meet her on a regular basis, I'll just have to keep making slow, but steady progress. As long as I trigger the right flags, I should be able to get the ending I want.