Musannaf Ibn Abi Shayba 13:1384

Abu Bakr narrated to us, he said: Azhar narrated to us, from Ibn ‘Awn, who said: Ibn Sirin used to like, when a *mukātab* (a slave under a contract of manumission) was writing the contract, that he should write in the contract: "And deduct [a portion] from the last installment of your installments."