Musannaf Ibn Abi Shayba 13:1788

Abu Bakr narrated to us, he said: Isa bin Yunus narrated to us, from al-Awza’i, who said: I purchased a slave girl and stipulated to her people that I would not sell her, nor give her away, nor give her as a dowry, and that if I died, she would be free. So I asked al-Hakam bin Utaybah, and he said: "There is no harm in it." And I asked the freed slave of Ata—or he was asked—and he disliked it. Al-Awza’i said: Yahya bin Abi Kathir narrated to me, from al-Hasan, who said: "The sale is permissible, and the condition is void." And I asked Abdah bin Abi Lubabah, and he said: "This is a bad private part." And I asked al-Zuhri, and he informed me that Ibn Mas’ud wrote to Umar asking him about a slave girl he had purchased from his wife on the condition that if he sold her, she would have more right to her for the price. Umar said: "Do not have intercourse with a private part that has a claim in it for someone else."