A man granting his slave or slave woman a contract of manumission in two or more instalments for valid wealth

Sayyidah Aishah (may Allah be pleased with her) says that Barirah came to her and said: My masters have made a contract of manumission (mukatabah) with me for nine uqiyahs over a period of nine years, one uqiyah each year, so help me.
(b) Sayyiduna Abu Hurayrah (may Allah be pleased with him) narrates from the Prophet (peace and blessings be upon him) that he (peace and blessings be upon him) forbade "the sale involving uncertainty (gharar)."

Muslim bin Abi Maryam narrates from a man who said: I was a slave of Uthman (may Allah be pleased with him). Uthman (may Allah be pleased with him) sent me on a trading venture; when I returned to him, he praised my management. He says: One day I stood before him and said: "O Commander of the Faithful, I ask you for a contract of manumission (mukatabah)." He frowned and said: "Yes; were it not for a verse in the Book of Allah, I would not do it. I will make the contract with you for one hundred thousand, on condition that you pay it to me in two instalments; by Allah, I will not reduce it by a single dirham." He says: I left him and Zubayr bin al-Awwam (may Allah be pleased with him) met me. He said: "What is the matter with you that I see?" I said: The Commander of the Faithful had sent me on a trading venture; when I returned to him he praised my management, so I stood before him and said: "O Commander of the Faithful, I ask you for a contract of manumission." He frowned and said: "Yes; were it not for a verse in the Book of Allah, I would not do it. I will make the contract with you for one hundred thousand, on condition that you pay it to me in two instalments; by Allah, I will not reduce it by a single dirham." Zubayr said: "Come along." He took me back to him, stood before him and said: "O Commander of the Faithful, have you made a contract with so-and-so?" He frowned and said: "Yes; were it not for a verse in the Book of Allah, I would not have done it. I am making the contract with him for one hundred thousand, on condition that he pays it to me in two instalments; by Allah, I will not reduce it by a single dirham." The narrator says: Zubayr (may Allah be pleased with him) became angry and said: "By Allah, I will stand before you as a petitioner! I only ask a need of you, and you bar it with an oath?" He says: Then he struck, I do not know whether he said my shoulder or my upper arm, and then said: "Make the contract with him." He says: So I made the contract with him, and Zubayr bin al-Awwam (may Allah be pleased with him) took me to his family and gave me one hundred thousand, then said: "Go and seek the bounty of Allah with it; if something overwhelms you, then pay Uthman what is due to him from it." He says: So I went and sought the bounty of Allah with it, and I paid Uthman (may Allah be pleased with him) what was due to him and Zubayr (may Allah be pleased with him) what was due to him, and eighty thousand remained in my hands.