Inherent Anti-Slave Social Mobility Dynamics

 You can kidnap a slave. However, slaves are fungible. Kidnapping is more trouble than buying one. 

 A slave will resist a kidnapping, but only because they like getting punched. They like oppression and tyranny, meaning a violent kidnapping is something they appreciate. They won't try to escape their kidnapper, or try to sabotage any tasks they're set. Or rather, slaves usually try to escape and sabotage (to provoke whippings, not to succeed), and will do so slightly less with the kidnapper since they see the violent kidnapper as more mastery than their previous boring master. Slaves dig jerks.

 Since slave kidnapping only occurs due to bouts of irrationality, anti-kidnapping security is light. Not worth the money. 

 This means if you've enslaved a non-slave, he is worth kidnapping. He is not available at the slave store, he is only available via kidnapping, and cattle rustlers will trivially rustle him.
 However, Revenge is Sour. The non-slave slave can negotiate for his freedom. "I will help you kidnap me, provided I'm manumitted. I agree to do [task] or [mission] in return." It's normal for slave owners to intuitively grasp this. Because the non-slave slave can negotiate with potential kidnappers, he can also negotiate with his original owner. Because he can negotiate with his original owner, he doesn't have to. 

 Slavers normally don't try to enslave non-slaves in the first place. They'll come up with all sorts of confabulating rot to explain the choice, but you can ignore their words. This game has a nash equilibrium, and it's the non-slave not being a slave. E.g. fake-stoic epictetus, even held back by genetic deviancy, just simply stopped being a slave at one point. "I'ma not be a slave now." "K, cool." Game over.

 Security is affordable. Non-slave slaves own their freedom, and can afford to secure it, meaning the alleged slave owner cannot afford to secure non-slave slaves. The fake slave's work output is superior, but not so superior he can pay for the jail costs required to keep him from arranging his own kidnapping, even setting aside the ruinous response to attempting such tactics on an honourable man. If his new owner welches on the deal and refuses to manumit him, he simply arranges for a second kidnapping. The world is just. 

 Some tried to enslave julius kaisar. The result was getting burned to death. Suicide with extra steps.
 Meanwhile, if you try to forcibly free a natural slave, they end up [[voluntarily]] working in slavelike conditions for slave wages, if they're lucky. If you can no longer whip the slave, they will work less, meaning they can't be paid as highly as their previous wage of zero.