Qasim narrates from Sayyidah Aishah (may Allah be pleased with her) that she intended to buy Barirah in order to free her, and her people stipulated the wala'. Sayyidah Aishah (may Allah be pleased with her) mentioned this to the Prophet (peace and blessings be upon him), and he (peace and blessings be upon him) said: "Buy her and free her, for the wala' belongs to the one who frees."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21733