Urwah (may Allah have mercy on him) said that Aishah (may Allah be pleased with her) informed him that Barirah (may Allah be pleased with her) sought the help of Aishah (may Allah be pleased with her) with her kitabah (contract of manumission), and she had not yet paid anything of it. Aishah (may Allah be pleased with her) said: Go; if they agree, I will pay off your kitabah and the wala' will be mine. Then Barirah (may Allah be pleased with her) mentioned it (to them), but they refused and said: If she seeks the reward, let her do it, but the wala' will be ours. She mentioned it to the Prophet (peace and blessings be upon him), and the Prophet (peace and blessings be upon him) said: "Buy and set free; the wala' belongs to the one who frees." Then the Messenger of Allah (peace and blessings be upon him) stood up and said: "What is wrong with people who stipulate conditions that are not in the Book of Allah! Whoever stipulates a condition that is not in the Book of Allah, it has no validity, even if they stipulate a hundred conditions. The conditions of Allah are more deserving of being fulfilled."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الولاء / 21472