Musannaf Ibn Abi Shayba 13:2099

Abu Bakr narrated to us, he said: Abu Khalid al-Ahmar narrated to us, from Shu'bah, from al-Hakam, from Abu Ja'far, he said: The Prophet, may Allah bless him and grant him peace, sold the service of the *mudabbar* (a slave promised freedom upon the master's death).