Return to your people ; if you like that I make payment for the purchase of your freedom on your behalf and I shall have the right to inherit from you, I shall do so. Barirah mentioned it to her people, but they refused and said: If she wants to purchase your freedom for reward from Allah, she may do so, but the right to inherit from her shall be ours. She mentioned it to the Messenger of Allah ﷺ. The Messenger of Allah ﷺ said: Purchase her (freedom) and set her free, for the right of inheritance belongs to only to the one who set a person free. The Messenger of Allah ﷺ then stood up and said: If anyone makes a condition which is not in Allah's Book, he has no right to it, even if he stipulates it hundred times. Allah's condition is more valid and binding. (Using translation from Abū Dāʾūd 3929)   

يَا عَائِشَةُ إِنِّي كَاتَبْتُ أَهْلِي عَلَى تِسْعِ أَوَاقٍ فِي كُلِّ عَامٍ وُقِيَّةٌ فَأَعِينِينِي وَلَمْ تَكُنْ قَضَتْ مِنْ كِتَابَتِهَا شَيْئًا فَقَالَتْ لَهَا عَائِشَةُ وَنَفِسَتْ فِيهَا ارْجِعِي إِلَى أَهْلِكِ فَإِنْ أَحَبُّوا أَنْ أُعْطِيَهُمْ ذَلِكَ جَمِيعًا وَيَكُونَ وَلَاؤُكِ لِي فَعَلْتُ فَذَهَبَتْ بَرِيرَةُ إِلَى أَهْلِهَا فَعَرَضَتْ ذَلِكَ عَلَيْهِمْ فَأَبَوْا وَقَالُوا إِنْ شَاءَتْ أَنْ تَحْتَسِبَ عَلَيْكِ فَلْتَفْعَلْ وَيَكُونَ ذَلِكَ لَنَا فَذَكَرَتْ ذَلِكَ عَائِشَةُ لِرَسُولِ اللهِ ﷺ فَقَالَ «لَا يَمْنَعُكِ ذَلِكَ مِنْهَا ابْتَاعِي وَأَعْتِقِي؛ فَإِنّمَا الْوَلَاءُ لِمَنْ أَعْتَقَ» فَفَعَلَتْ وَقَامَ رَسُولُ اللهِ ﷺ فَحَمِدَ اللهَ ثُمَّ قَالَ «أَمَّا بَعْدُ فَمَا بَالُ أُنَاسٍ يَشْتَرِطُونَ شُرُوطًا لَيْسَتْ فِي كِتَابِ اللهِ مَنِ اشْتَرَطَ شَرْطًا لَيْسَ فِي كِتَابِ اللهِ فَهُوَ بَاطِلٌ وَإِنْ كَانَ مِائَةُ شَرْطٍ قَضَاءُ اللهِ أَحَقُّ وَشَرْطُ اللهِ أَوْثَقُ وَإِنَّمَا الْوَلَاءُ لِمَنْ أَعْتَقَ»