Musannaf Ibn Abi Shaybah

Ismail (may Allah have mercy on him) says that I asked Hazrat Ata (may Allah have mercy on him) what is the ruling if a mistress says to her slave, "I free you on the condition that you marry me?" He replied that it would have been better if she had simply freed him. I asked Hazrat Abdullah bin Ubaid bin Umair (may Allah have mercy on him) the same question, and he gave a similar answer as Hazrat Ata (may Allah have mercy on him). I asked Hazrat Mujahid (may Allah have mercy on him), and he became angry and said that this is an act punishable by Allah and the ruler.

Hazrat Ata (may Allah have mercy on him) and Hazrat Abdullah bin Ubaid bin Umair (may Allah have mercy on him) say that he should set him free but should not stipulate a condition, and Hazrat Mujahid (may Allah have mercy on him) says that this is a cause for punishment from Allah and the king.