Hadith: Loyalty is pledged to the one who did the favor

عن عائشة رضي الله عنها أنها اشترتْ بَريرَة مِنْ أُناسٍ من الأنصار واشْتَرَطوا الوَلاء، فقال رسول الله صلى الله عليه وسلم : «الوَلاءُ لِمَنْ وَلِيَ النِّعْمَة»، وخَيَّرَهَا رسول الله صلى الله عليه وسلم وكان زوْجُها عبْدا، وأهدَتْ لعائشة لحْما، فقال رسول الله صلى الله عليه وسلم : «لو صَنَعْتُم لنا من هذا اللحم»، قالت عائشة: تُصُدِّقَ به على بَرِيرَة، فقال: «هو لها صدَقة ولنا هديَّة».
[صحيح] - [متفق عليه]

‘Ā’ishah (may Allah be pleased with her) bought Barīrah from a household belonging to the Ansār, and they stipulated that they retain the ex-slave's allegiance. The Messenger of Allah (may Allah’s peace and blessings be upon him) said: "Allegiance is pledged to the one who did the favor (of setting the slave free)." The Prophet (may Allah's peace and blessings be upon him) also gave Barīrah the choice (either to retain her marriage or break it), as her husband was a slave. She gave some meat to ‘Ā’ishah as a gift, and the Messenger of Allah (may Allah’s peace and blessings be upon him) said: "I wish you could cook this meat for us." ‘Ā’ishah said: "It was given in charity to Barīrah." He said: "It was charity to her but a gift to us."
Sahih/Authentic. - [Al-Bukhari and Muslim]

Explanation

According to this Hadīth, ‘Ā’ishah (may Allah be pleased with her) bought Barīrah and freed her, but her ex-master conditioned that they retain Barīrah's allegiance. The Messenger of Allah (may Allah’s peace and blessings be upon him) told her that this condition is void because allegiance is pledged to the one who did the favor of setting the slave free. Regarding Barīrah, she was married to a slave named Mughīth, so when she became higher than him (in rank) as a free woman with an independent will, the Prophet (may Allah's peace and blessings be upon him) gave her the choice either to retain her matrimonial bond or break it. Later, Barīrah received meat as charity and sent some of it to ‘Ā’ishah (may Allah be pleased with her) as a gift. The Messenger of Allah (may Allah’s peace and blessings be upon him) wanted to eat from that meat. ‘Ā’ishah (may Allah be pleased with her) told the Prophet (may Allah's peace and blessings be upon him) that it had originally been given in charity to Barīrah, and thus he was not allowed to eat from it. However, the Prophet (may Allah's peace and blessings be upon him) told her that Barīrah was given the meat as charity, while the Prophet (may Allah's peace and blessings be upon him) was given it as a gift from her. Therefore, the ruling of this meat changed from a charity to a gift, so eating it was allowable for the Prophet (may Allah's peace and blessings be upon him) and his household.