But is there any one thus intended by nature to be a slave, and for whom such a condition is expedient and right, or rather is not all slavery a violation of nature?
Without sociopathic tendencies most people need to dehumanize someone in order to dominate them for slavery to properly function. In Aristotle's lifetime the slaves were barbarians. Barbarians were considered anyone who wasn't of an educated Greek order. Whether it's religion, race, or class distinction the ability to place the slave class in a category of being other is not just a postulate, but a constitutive part of justifying the immorality of the institution.
The three dominant western religions all have scriptures that can be used for the justification of slavery. When you add religious approval to in-group out-group bias, you have a moral slip n slide in which the use of humans as tools for production is acceptable.
This explains the subjugation of people, but does nothing to deal with the question of someone being a "born slave". I'm comfortable knowing there's no serious conversation about moving our culture back to a place where people, who have a predisposition for servitude, could be easily exploited by individuals or market factors. We should strive for a society in which we actively prevent the exploitation of people, even those who may even want it.