Musannaf Ibn Abi Shayba 18:605

Abu Bakr narrated to us, he said: Ibn ‘Ulayyah narrated to us, from Yunus, from al-Hasan, regarding the crime of the *mudabbar* (a slave promised freedom upon the master's death): He said: He is a slave; if his master wishes, he may surrender him, and if he wishes, he may ransom him.