Abu Sa'id (may Allah be pleased with him), the freed slave of Abu Usayd (may Allah be pleased with him), (narrates that) Abu Usayd made a kitabah contract with him, his slave, for one thousand two hundred dirhams. He says: I came with my kitabah amount, and he returned two hundred dirhams to me.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21676