Imam al-Shafi'i (may Allah have mercy on him) said: The hadith of Yahya from Amrah from Sayyidah Aishah (may Allah be pleased with her) is more established than the hadith of Hisham, and I think he erred in his words: "Stipulate the wala' for them." I understand the hadith of Amrah to mean that Sayyidah Aishah (may Allah be pleased with her) had stipulated that for them without the command of the Prophet (peace and blessings be upon him), thinking it permissible, so the Messenger of Allah (peace and blessings be upon him) informed her that if she freed her, the wala' would be hers, and said: "Let the condition you made earlier not keep you from her." And I do not think that he ordered her to stipulate for them what is not permissible.
The hadith of Amrah from Sayyidah Aishah (may Allah be pleased with her) is an established hadith; a group has narrated it from Yahya bin Sa'id with a connected chain.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21725