The Messenger of Allah ﷺ forbade selling or giving away the right to inheritance by a manumitted slave. (Using translation from Abū Dāʾūd 2919)  

«نَهَى رَسُولُ اللهِ ﷺ عَنْ بَيْعِ الْوَلَاءِ وَعَنْ هِبَتِهِ»