ولا يخرج عن مكاتبه ولاعن مماليكه للتجارة
Translation: He does not pay on behalf of his mukatab or slaves held as trading stock.
A mukatab was an enslaved person with a contractual arrangement to obtain freedom; the second category comprises those held as saleable stock in the historical legal system.
Read this beside the service-slave rule: the original juristic system did not treat every form of ownership alike for fitrah liability.