Maybe have the master pay an extra fee for the convenience of his slave being able to travel easily with him when he goes on glorious hunts or what not.
Or an item that only the master can purchase and hand to their slave or "use" on their slave before using the portal to ensure said slave can't use it without permission?
An untradeable item that is purchased by the one holding the slaves' papers and only tradable to that PC so the slave can travel with others who might have gained IC permissions to use the masters' property for their own means?
I'm sure you get the idea I'm going for here.
