Hey I'm having trouble finding a link to a story that will open it. I don't know for sure if it is on here. It's called The Swap and it has a follow up story called aftermath.
The story is about Buffy and Riley attending a party at Riley's Boss's house (AKA Angel). Angel sees Buffy and wants her for himself. I tells the men in the party to get their dates driver's license and they go into a bowl and Angel purposely gets Buffy's name. They would have to spend the night together for the swap. Buffy gets mad at Riley because he was willing to do it to get a promotion at work.
Thanks