Musannaf Ibn Abi Shaybah

A slave whom the master has permitted to engage in trade, then he becomes indebted and his master also sets him free. Regarding this slave, Hammad (may Allah have mercy on him) says that his master will be liable for the price of this slave. And Sufyan (may Allah have mercy on him) says that his creditors will continue to pursue him for the remaining debt until he pays it off.