(3) باب الشُّرُوطِ فِي الْبَيْعِ
Narrated `Urwa:
Aisha told me that Buraira came to seek her help in writing for emancipation and at that time she had not paid any part of her price. `Aisha said to her, "Go to your masters and if they agree that I will pay your price (and free you) on condition that your Wala' will be for me, I will pay the money." Buraira told her masters about that, but they refused, and said, "If `Aisha wants to do a favor she could, but your Wala will be for us." Aisha informed Allah's Messenger (ﷺ) of that and he said to her, "Buy and manumit Buraira as the Wala' will go to the manumitted."
Reference: Sahih al-Bukhari 2717
In-book reference: Book 54, Hadith 6
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 3, Book 50, Hadith 878