Can a Mukatab (contracted slave) appoint an agent for matters he can manage himself?

Primary text

A Mukatab is permitted to appoint an agent concerning matters within his own capacity to conduct. Furthermore, he may accept agency (*tawkil*) for a fee (*ja'l*), as this constitutes earning wealth. The Mukatab is not prevented from earning. However, he cannot act as an agent for others without compensation unless granted permission by his master, because his services are treated like the essence of his property, and he cannot give away the essence of his property without receiving compensation.