The Messenger of Allah ﷺ said to us: If one of you has a slave, and he enters into an agreement to purchase his freedom, and can pay the full price, she must veil herself from him. (Using translation from Abū Dāʾūd 3928)

"إذا كانَ لإِحْداكُنَّ مُكَاتَبٌ، فكانَ عِنْده ما يُؤَدِّي فليحتَجِبْ مِنْهُ" .