Musannaf Ibn Abi Shayba 13:1554

Abu Bakr narrated to us, he said: Jarir narrated to us, from Mansur, from Ibrahim, who said: If there is any surplus from the property of a mukatab (an enslaved person under a contract of manumission), it belongs to his masters until his contract of manumission is completed. And if there is any surplus after his contract of manumission is completed, it belongs to his heirs.