Musannaf Ibn Abi Shaybah

Hazrat Ibn Masood (may Allah be pleased with him) disliked the sale of a Mukatab (a slave who has a contract for freedom).

Ibrahim says that if the amount of the contract of emancipation (kitabah) remains due upon a mukatab, and he is sold to the one from whom he was bought, and he is made responsible for his own emancipation, then there is no harm in it; he will not be sold for the purpose of slavery.

It is narrated from Hazrat Aisha (may Allah be pleased with her) that Hazrat Barirah, who was a Mukatabah (a slave woman with a contract of emancipation), came to me, so I asked the Prophet (peace and blessings be upon him) that if I buy her, will her Wala (right of inheritance/loyalty) go to her former master? The Prophet (peace and blessings be upon him) said: Buy her and set her free; Wala belongs to the one who sets free.