Musnad Abu Ya'la al-Mawsili 108:58

Abu Khaythama narrated to us, Sufyan ibn 'Uyaynah narrated to us, he heard 'Amr [narrating] from Jabir ibn 'Abd Allah: That the Prophet (peace and blessings of Allah be upon him) sold a *mudabbar* (a slave promised freedom upon the master's death).