I can't remember who wrote this or the title:
Buffy is a trained dancer who took out a loan either for Joyce or a dance studio through Angel's nefarious connections and owes a loan shark big time. She actually lost her virginity to Spike when he was drunk and on a break with Dru. He pays for a private dance and tries to get to know Buffy better.

Sorry, folks! It's a greyangel fic, "Dancing in the Dark." Should've remembered that it was offline.