Musannaf Ibn Abi Shayba 28:335

Al-Fadl ibn Dukayn narrated to us, he said: Abbad narrated to us, he said: Makhul informed me, he said regarding a *mukātab* (a slave who has entered into a contract of manumission) who was captured by the enemy, then a merchant bought him and entered into a *mukātaba* (contract of manumission) with him: He shall fulfill the *mukātaba* of the first, then he shall fulfill the *mukātaba* of the second.