So I was thinking this was an Asheligh Kinnerson (not Stepping Out of the Dark) or Moonshadow Woman story, but I can't find it.
Justin is a specially trained [sex] slave that Brian wins in a Poker game. This is truly slavery - as in the people are bought and sold, not a domination sex slave thing. There are rules about how these slaves can be treated. Brian ends up fighting for the rights and emancipation of Justin and other slaves like him. They aren't called slaves.
Pretty sure it's a Moonshadow Woman fic - checked tags, but didn't find it.
ETA: found by
amythesw who knew it was Companion by Julesmonster?
https://www.fanfiction.net/s/8140090/1/Companion