I imagine that Slavery would evolve to something more like Modern Dubai or Qatar where people are on paper free but have work permits that limit where they can work or move, with ownership of said permits being bought, rented or sold between individual industries.

Say you are Cletus and want to start a factory, you build the place, start shipment of raw material, and then to start the operations, you rent out 200 work permits some management firm, whose "workers" are then instructed to move to your factory