Musnad Ahmad 33:817

Al-Fadl ibn Dukayn narrated to us, Sharik narrated to us, from Salamah—meaning ibn Kuhayl—from ‘Ata’ and Abu al-Zubayr, from Jabir: "A man died and left behind a *mudabbar* (a slave promised freedom upon the master's death) and a debt. The Messenger of Allah, peace and blessings of Allah be upon him, ordered them to sell him for his debt, so they sold him for eight hundred."