That makes NO sense. Just because you paid the thief for a stolen car does not give you the right to keep it. Similarly, just because you didn’t personally abduct somebody from their home does not give you the moral right keep them enslaved. And the importation of new slaves into the US was made illegal in 1818, so by the Civil War, very few of the slaves in the US were “already slaves in Africa.”
You know I like the modern practice of replacing the terms slaves and slave owners (holders) with the enslaved people and enslavers. It emphasized the fact that keeping people enslaved and torturing them so that they work for you is an ACTIVE process. Torture and the threat of torture was really the only thing that kept many from trying to secure their freedom by running away.