Ok, I know that I've read this fic, but I can't seem to find anything through google-ing (I must not have the exact phrases correct).
Spike asks Xander if he can turn him, and Xander says that he has to think about it. Later, they get into a huge fight and Xander tells Spike that he can't turn him, and Spike says something like "that means I'll lose you" and Xander threatens to kill him if he does turn him.
Anyone have any clue?
FOUND: Repossession by Lazulikat, Chapter 46.