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.

لَنَا رَسُولُ اللَّهِ ﷺ إِنْ كَانَ لإِحْدَاكُنَّ مُكَاتَبٌ فَكَانَ عِنْدَهُ مَا يُؤَدِّي فَلْتَحْتَجِبْ مِنْهُ