Musannaf Ibn Abi Shaybah

Hasan (may Allah have mercy on him) said that if a slave conducts trade without the permission of his master and becomes indebted, then he is set free, indeed he will be sold for that debt.

Hazrat Zuhri (may Allah have mercy on him) says that if a slave buys or sells without the permission of his master, nothing will be obligatory upon the master; everything will be the responsibility of the slave, and when he is freed, everything will remain obligatory upon him.

Hazrat Hammad, may Allah have mercy on him, was asked: If a slave buys and sells without the permission of his master, and then he is set free, he replied: Nothing will be obligatory upon the master; the debts of the creditors will remain upon the slave when he is set free.