Abu Qilabah (may Allah have mercy on him) says that the wives of the Messenger of Allah (peace and blessings be upon him) (may Allah be pleased with them) did not observe hijab from their mukatab slave as long as even one dinar remained due from him.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21648