Nafi' (may Allah have mercy on him) says that Sayyidah Hafsah (may Allah be pleased with her), the wife of the Prophet (peace and blessings be upon him), made a contract of manumission (mukatabah) with a slave of hers in exchange for slaves. Nafi' (may Allah have mercy on him) says: I myself met three of those who were paid in their contract of manumission.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21628