Haha, it happens! I know it seems a little counter-intuitive for the Sith/Empire to be anti-slavery, but the old Dark Lord was a slave as a kid iirc, so once he got all powerful he abolished it across all territories. #themoreyouknow
I'm actually a bit infuriated at a few things, so I'ma do my due diligence and drop-smoke and ninja vanish (because slipping out of chaos and into the ether rights several wrongs) and make a bit of a decision on something for my character that I'll retcon in the profile later.
If you're still eager for the character to end up his slave, in theory there's nothing actually stopping you. The issue just arises IC when and if he gets caught dragging her around. Y'know, in-character consequences and all that.
Big picture is important with me, and as I've told someone else, I need, hell I dunno, something challenging, something gritty, and something that actually causes character development, not something where I can join a thread and autopilot.
My advice? She seems to be a better fit for the Hutt Cartel. Being a slave and all that, but presumably a character who ends up doing more than being a dancer. Obviously, that depends on what you wanted to do with her Force sensitivity. Otherwise, she doesn't end up his slave, but gets brought into the Sith as a 'free' Acolyte. More or less the same result, just... well, not called a slave.