Makhramah bin Bukayr narrates from his father that Nafi' said: Abdullah bin Umar (may Allah be pleased with them both) made a kitabah contract with his slave, fixing 35 thousand dirhams, and reduced 5 thousand dirhams from the last part of his kitabah. Nafi' did not mention whether he gave him anything besides the amount reduced.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21674