There came a slave and pledg- ed allegiance to Allah's Apostle ﷺ on migration; he (the Holy Prophet) did not know that he was a slave. Then there came his master and demanded him back, whereupon Allah's Apostle ﷺ said: Sell him to me. And he bought him for two black slaves, and he did not afterwards take allegiance from anyone until he had asked him whether he was a slave (or a free man)  

جَاءَ عَبْدٌ فَبَايَعَ النَّبِيَّ ﷺ عَلَى الْهِجْرَةِ وَلَمْ يَشْعُرْ أَنَّهُ عَبْدٌ فَجَاءَ سَيِّدُهُ يُرِيدُهُ فَقَالَ لَهُ النَّبِيُّ ﷺ بِعْنِيهِ فَاشْتَرَاهُ بِعَبْدَيْنِ أَسْوَدَيْنِ ثُمَّ لَمْ يُبَايِعْ أَحَدًا بَعْدُ حَتَّى يَسْأَلَهُ أَعَبْدٌ هُوَ