Musannaf Ibn Abi Shayba 13:2269

Abu Bakr narrated to us, he said: Waki’ narrated to us, he said: Al-Rabi’ narrated to us, he said: Umar ibn Abd al-Aziz wrote to us: "A man may take from his mukatab (slave under a contract of manumission) trade goods (as payment for his manumission)."