Amrah narrates from Sayyidah Aishah (may Allah be pleased with her) that Barirah (may Allah be pleased with her) came to Sayyidah Aishah (may Allah be pleased with her) to seek help with her contract of manumission. Sayyidah Aishah (may Allah be pleased with her) said: "If your masters wish, I will pay the whole price at once and free you." Barirah (may Allah be pleased with her) mentioned this to her people, and they said: (Only) with the condition of the wala'. Sayyidah Aishah (may Allah be pleased with her) mentioned this to the Messenger of Allah (peace and blessings be upon him), and he (peace and blessings be upon him) said: "Buy her, for the wala' belongs to the one who frees."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21726