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)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21622