First is a SciFi Spike/Xander fic. Set in an Alternate Universe. Spike Angel are Genetically different from humans. What I can remember is Xander gets captured by Spike when Spike raids a ship for supplies. Xander is someone important and close to Buffy. Willow is a something like a Techno-Mage. After Xan gets captured he falls for Spike, and Spike for him. After an outlaw faction attacks a human outpost Xander returns to Buffy to try and rescue Spike. Also Xander has been changed. Buffy and Willow and Tara learn that Giles and the Council have been lying to control the Slayer line, breeding them to be obedient to their watcher. Xander reveals that he has been changed when the truth about Giles and Council lying and Willow helps him and Spike escape. Hope that is enough to go on.
The Second is Fantasy. Spike is sold by his people into slavery. Xander is trying to find a warrior that can help his people fight and survive and sees Spike on the slave auction and he buys him to save him from being a sex toy. But one of the conditions is that Spike has to be bound to him. Because of the language barrier Spike does not realize that Xander only wanted to help and thinks that he has been bought as a toy. Xander gives control over to Spike and Spike hurts him pretty badly. Xander gives up his future plans of marriage because if Spike cant be free then he wont be either. Know its kind of vague but hope it clicks with someone.