Amr bin Shu'ayb narrates from his father, from his grandfather (may Allah be pleased with them both), that the Messenger of Allah (peace and blessings be upon him) delivered a sermon and said: "Any man who makes a contract of manumission (mukatabah) with his slave for one hundred uqiyahs, if the slave is unable to pay ten uqiyahs, he remains a slave."
Al-Shafi'i (may Allah have mercy on him) said in his old (school): I do not know anyone who narrated this from the Prophet (peace and blessings be upon him) except Amr bin Shu'ayb, and the fatwa of the muftis is according to this.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21639