Muhammad bin Sirin (may Allah have mercy on him) says regarding this saying, "And give them from the wealth of Allah which He has given you" [Surah al-Nur: 33], that they liked a man to leave some part of the kitabah of his mukatab slave.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21679