Musannaf Ibn Abi Shaybah

Hazrat Sufyan says that if a person frees a slave, the slave will be released from mortgage, and if he makes him a mudabbar (a slave who will be freed after the master's death), he will also be released from mortgage. And if it is a bondwoman and he has intercourse with her and she bears a child, she too will be released from mortgage. Then, if the master is wealthy, the mortgagee will seize the master, and if the master is poor, these people (the slave and the bondwoman) will strive for the price and mortgage, whichever is less in value. Hazrat Sufyan says that then the slave will refer to his master for as much as he has strived (i.e., he will receive that much money or value from him), but umm walad (a bondwoman who has borne a child) and mudabbar will not refer to the master because their service is for the master.