Musannaf Ibn Abi Shaybah

Hazrat Hakam (may Allah have mercy on him) says that if a slave is such that he pays the fixed portion of kharaj, and he makes a sale, then the transaction is upon his own neck. Hazrat Hammad (may Allah have mercy on him) says that if the master has given him permission to sell, then it is upon the master's neck.

Hazrat Harith (may Allah be pleased with him) and Hazrat Hammad (may Allah have mercy on him) state that if the owner separates himself from the slave on a fixed tribute, then this is granting him permission to sell.