Tammy: I worked it off classical slavery in general; captives from wars, then the children they give birth to. And most slave systems (except the modern ones) have the pressure valve that the slave can buy her/his freedom. It’s a carrot, because slave holders almost always find ways to keep slaves from accumulating their price, and there are the added anchors of spouses and children who are slaves.
Palace slaves in Carthak could make money toward their freedom price on tips for running errands or good service, but they can also lose that money to fines and theft. But it pays to keep slaves hoping that one day they can buy freedom–they’re less inclined to make trouble that way. Even slaves here in the U.S. could buy their freedom. They then had a set period of time (I forget how long) to get out of the state in which they’d been freed, to reduce the temptation of “stealing” their families and to get rid of the reminder that it was possible to be free. Free blacks who had not been freed locally were allowed to work in towns, but they were scrutinized and suspected constantly, and they always had to carry multiple sets of papers proving they were free to keep from being captured and sold.