If a Mukatab slave asks somebody to buy and free him

Narrated 'Abdul Wahid bin Aiman:

I went to 'Aisha and said, "I was the slave of Utba bin Abu Lahab. "Utba died and his sons became my masters who sold me to Ibn Abu Amr who manumitted me. The sons of 'Utba stipulated that my Wala' should be for them." 'Aisha said, "Buraira came to me and she was given the writing of emancipation by her masters and she asked me to buy and manumit her. I agreed to it, but Buraira told me that her masters would not sell her unless her Wala' was for them." 'Aisha said, "I am not in need of that." When the Prophet heard that, or he was told about it, he asked 'Aisha about it. 'Aisha mentioned what Buraira had told her. The Prophet ﷺ said, "Buy and manumit her and let them stipulate whatever they like." So, 'Aisha bought and manumitted her and her masters stipulated that her Wala' should be for them." The Prophet;
, said, "The Wala' will be for the liberator even if they stipulated a hundred conditions."

حَدَّثَنَا أَبُو نُعَيْمٍ، حَدَّثَنَا عَبْدُ الْوَاحِدِ بْنُ أَيْمَنَ، قَالَ حَدَّثَنِي أَبِي أَيْمَنُ، قَالَ دَخَلْتُ عَلَى عَائِشَةَ ـ رضى الله عنها ـ فَقُلْتُ كُنْتُ غُلاَمًا لِعُتْبَةَ بْنِ أَبِي لَهَبٍ، وَمَاتَ وَوَرِثَنِي بَنُوهُ، وَإِنَّهُمْ بَاعُونِي مِنَ ابْنِ أَبِي عَمْرٍو، فَأَعْتَقَنِي ابْنُ أَبِي عَمْرٍو، وَاشْتَرَطَ بَنُو عُتْبَةَ الْوَلاَءَ. فَقَالَتْ دَخَلَتْ بَرِيرَةُ وَهْىَ مُكَاتَبَةٌ فَقَالَتِ اشْتَرِينِي وَأَعْتِقِينِي. قَالَتْ نَعَمْ. قَالَتْ لاَ يَبِيعُونِي حَتَّى يَشْتَرِطُوا وَلاَئِي. فَقَالَتْ لاَ حَاجَةَ لِي بِذَلِكَ. فَسَمِعَ بِذَلِكَ النَّبِيُّ ﷺ أَوْ بَلَغَهُ، فَذَكَرَ لِعَائِشَةَ، فَذَكَرَتْ عَائِشَةُ مَا قَالَتْ لَهَا، فَقَالَ " اشْتَرِيهَا وَأَعْتِقِيهَا، وَدَعِيهِمْ يَشْتَرِطُونَ مَا شَاءُوا ". فَاشْتَرَتْهَا عَائِشَةُ فَأَعْتَقَتْهَا وَاشْتَرَطَ أَهْلُهَا الْوَلاَءَ، فَقَالَ النَّبِيُّ ﷺ " الْوَلاَءُ لِمَنْ أَعْتَقَ، وَإِنِ اشْتَرَطُوا مِائَةَ شَرْطٍ ".