Musannaf Ibn Abi Shayba 13:2209

Abu Bakr narrated to us, he said: Ibn Idris narrated to us, from Hisham, from al-Hasan, who said: If a man’s bankruptcy becomes evident, his manumission [of a slave] is not permissible while he has debt. But if his bankruptcy has not become evident, his manumission is permissible.