I have loaned women substantial amounts of money four times in my life. Two times I had not been sleeping with the woman; those two times I got paid back with no hassles.
Two times it was to a girlfriend or ex-wife, and those two times turned out very badly indeed. Once I finally had to file small claims papers to get repaid, the other I only saw half the money returned after ten years had gone by. (No lie. Ten years.)
I hate to say it, and I mean no disrespect to any of the women who post here, but I honestly believe that if a woman has slept with you, in her mind there’s something there that says, “He owes me something.” This had been my experience, and the experience of several of my friends. Older and wiser now, I will not make this mistake again.
I totally understand where you’re coming from. Trust me, you will not be able to let this go emotionally, so you should do something about it. Talk to her parents, threaten small claims. Tell your ex (very dispassionately) that you’re not going to take an $800 kick in the nuts and that if she doesn’t cough up the rest of the money you WILL file. Give her a deadline. If she gets all “I can’t believe that you’re making a big deal out of this” on you, just shrug and tell her you can’t afford to lose the $800. Don’t get angry, don’t let your emotions into it. Just tell her. Act like you simply don’t have any choice.