Abd al-Rahman bin al-Qasim (may Allah have mercy on him) narrates from his father (may Allah have mercy on him) that the Mothers of the Believers (may Allah be pleased with them) did not observe hijab from their mukatab slave as long as even one dirham remained due from him. When he paid off the money, they would begin observing hijab (from him).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21647