Sayyidah Umm Salamah (may Allah be pleased with her) says that the Messenger of Allah (peace and blessings be upon him) said: "When the mukatab of any one of you has the price with him, then she should observe hijab from him."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21661
Al-Zuhri said: Nabhan, the slave of Sayyidah Umm Salamah (may Allah be pleased with her), narrated to me; he said: I was leading her mount at al-Bayda' or al-Abwa'. She said: "Who is this?" I said: "I am Nabhan." She said: "I have left the remainder of your kitabah to my nephew Muhammad bin Abdullah bin Abi Umayyah; I have helped him with it in his marriage." Nabhan said: "By Allah! I will never pay it to him." Sayyidah Umm Salamah (may Allah be pleased with her) said: "If your intention is to see me, then by Allah! you will never be able to see me, for I heard the Messenger of Allah (peace and blessings be upon him) say: 'When the mukatab has with him the price he is to pay, then observe hijab from him.'"
(b) Amr bin Shu'ayb narrates from the Prophet (peace and blessings be upon him) that he (peace and blessings be upon him) said: "Whoever makes a kitabah contract with his slave for 100 uqiyyah, and only 10 uqiyyah remain, he is still a slave."
Al-Shafi'i (may Allah have mercy on him) said: I have not seen anyone among the people of knowledge whom I approve of affirming either of these two hadiths (the hadith of Nabhan and the hadith of Amr bin Shu'ayb). Al-Bukhari and Muslim did not record the hadith of Nabhan in the Sahih; it seems that his reliability was not established with them, or that he did not come out of the state of being unknown (majhul) through a trustworthy narrator narrating from him.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21663
Ibn Shihab (may Allah have mercy on him) says that Sayyidah Umm Salamah (may Allah be pleased with her), the wife of the Prophet (peace and blessings be upon him), sold Nabhan as her mukatab and said: "Give the remaining amount of your kitabah to my nephew Ibn Abdullah bin Abi Umayyah (may Allah be pleased with him); I want to help him. And from now on, speak to me only from behind a screen." Nabhan wept. Sayyidah Umm Salamah (may Allah be pleased with her) said that the Messenger of Allah (peace and blessings be upon him) said: "When one of you makes a kitabah contract with her slave, he may see her as long as some of the kitabah amount remains; when the amount has been paid in full, then speak only from behind a screen."
This is how Abdullah bin Ziyad bin Sam'an narrated it, and he is da'if (weak); the narration of the trustworthy narrators from al-Zuhri differs from it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21664