Selling A Mukatib

It was narrated from'Aishah that Barirah came to 'Aishah asking her to help her with her contract of manumission.' Aishah said:

"Go back to your masters, and if they agree to let me pay off your contract of manumission, and let your loyalty be to me, then I will do it" Barirah told her masters about that, but they refused and said: "If she wants to seek reward (with Allah) by freeing you, let her do so, but your loyalty will be to us." She told the Messenger of Allah about that , and the Messenger of Allah said to her; "Buy her and set her free, and loyalty belongs to the one who set the slave free," Then the Messenger of Allah said: "What is the matter with people who stipulate conditions that are not in the Book of Allah? Whoever stipulates something that is not in the Book of Allah, it is not valid even if he stipulates one hundred conditions? The condition of Allah is more deserving of being followed and is more hinting."

أَخْبَرَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، قَالَ حَدَّثَنَا اللَّيْثُ، عَنِ ابْنِ شِهَابٍ، عَنْ عُرْوَةَ، عَنْ عَائِشَةَ، أَخْبَرَتْهُ أَنَّ بَرِيرَةَ جَاءَتْ عَائِشَةَ تَسْتَعِينُهَا فِي كِتَابَتِهَا شَيْئًا فَقَالَتْ لَهَا عَائِشَةُ ارْجِعِي إِلَى أَهْلِكِ فَإِنْ أَحَبُّوا أَنْ أَقْضِيَ عَنْكِ كِتَابَتَكِ وَيَكُونَ وَلاَؤُكِ لِي فَعَلْتُ فَذَكَرَتْ ذَلِكَ بَرِيرَةُ لأَهْلِهَا فَأَبَوْا وَقَالُوا إِنْ شَاءَتْ أَنْ تَحْتَسِبَ عَلَيْكِ فَلْتَفْعَلْ وَيَكُونَ لَنَا وَلاَؤُكِ . فَذَكَرَتْ ذَلِكَ لِرَسُولِ اللَّهِ ﷺ فَقَالَ لَهَا رَسُولُ اللَّهِ ﷺ " ابْتَاعِي وَأَعْتِقِي فَإِنَّ الْوَلاَءَ لِمَنْ أَعْتَقَ " . ثُمَّ قَالَ رَسُولُ اللَّهِ ﷺ " مَا بَالُ أَقْوَامٍ يَشْتَرِطُونَ شُرُوطًا لَيْسَتْ فِي كِتَابِ اللَّهِ فَمَنِ اشْتَرَطَ شَيْئًا لَيْسَ فِي كِتَابِ اللَّهِ فَلَيْسَ لَهُ وَإِنِ اشْتَرَطَ مِائَةَ شَرْطٍ وَشَرْطُ اللَّهِ أَحَقُّ وَأَوْثَقُ " .