After discussing my plans for stealing or rather, rescuing (whichever is applicable) people from Ceryneia with Marius, I destroyed his slave certificate right in front of his eyes to show my sincerity about being his partner in crime instead of being his master. I prefer having loyal subordinates over having slaves that only say yes to everything I say and would not give me proper advise or insights on things. He may have a soul mark but that is just to keep his mouth shut about secrets, it doesn't function on making him follow whatever I say unconditionally.