Narrated 'Urwa:

That 'Aisha told him that Buraira came to seek her help in her writing of emancipation (for a certain sum) and that time she had not paid anything of it. 'Aisha said to her, "Go back to your masters, and if they agree that I will pay the amount of your writing of emancipation and get your Wala', I will do so." Buraira informed her masters of that but they refused and said, "If she (i.e. 'Aisha) is seeking Allah's reward, then she can do so, but your Wala' will be for us." 'Aisha mentioned that to Allah's Apostle who said to her, "Buy and manumit her, as the Wala' is for the liberator." Allah's Apostle then got up and said, "What about the people who stipulate conditions which are not present in Allah's Laws? Whoever imposes conditions which are not present in Allah's Laws, then those conditions will be invalid, even if he imposed these conditions a hundred times. Allah's conditions (Laws) are the truth and are more solid."

وَقَالَ اللَّيْثُ حَدَّثَنِي يُونُسُ، عَنِ ابْنِ شِهَابٍ، قَالَ عُرْوَةُ قَالَتْ عَائِشَةُ ـ رضى الله عنها ـ إِنَّ بَرِيرَةَ دَخَلَتْ عَلَيْهَا تَسْتَعِينُهَا فِي كِتَابَتِهَا وَعَلَيْهَا خَمْسَةُ أَوَاقٍ، نُجِّمَتْ عَلَيْهَا فِي خَمْسِ سِنِينَ، فَقَالَتْ لَهَا عَائِشَةُ وَنَفِسَتْ فِيهَا أَرَأَيْتِ إِنْ عَدَدْتُ لَهُمْ عَدَّةً وَاحِدَةً، أَيَبِيعُكِ أَهْلُكِ، فَأُعْتِقَكِ، فَيَكُونَ وَلاَؤُكِ لِي فَذَهَبَتْ بَرِيرَةُ إِلَى أَهْلِهَا، فَعَرَضَتْ ذَلِكَ عَلَيْهِمْ فَقَالُوا لاَ إِلاَّ أَنْ يَكُونَ لَنَا الْوَلاَءُ. قَالَتْ عَائِشَةُ فَدَخَلْتُ عَلَى رَسُولِ اللَّهِ ﷺ فَذَكَرْتُ ذَلِكَ لَهُ. فَقَالَ لَهَا رَسُولُ اللَّهِ ﷺ " اشْتَرِيهَا فَأَعْتِقِيهَا، فَإِنَّمَا الْوَلاَءُ لِمَنْ أَعْتَقَ ". ثُمَّ قَامَ رَسُولُ اللَّهِ ﷺ فَقَالَ " مَا بَالُ رِجَالٍ يَشْتَرِطُونَ شُرُوطًا لَيْسَتْ فِي كِتَابِ اللَّهِ، مَنِ اشْتَرَطَ شَرْطًا لَيْسَ فِي كِتَابِ اللَّهِ فَهْوَ بَاطِلٌ، شَرْطُ اللَّهِ أَحَقُّ وَأَوْثَقُ ".