"A slave came to give the pledge to the Prophet ﷺ for Hijrah, but the Prophet ﷺ did not realize that he was a slave. So his master came, and the Prophet ﷺ said: 'Sell him to me.' So he purchased him for two black slaves. After this he ﷺ would not take the pledge from anyone until he asked him if he was a slave." [He said:] There is something on this topic from Ibn 'Abbas.  

أَنَّهُ قَالَ جَاءَ عَبْدٌ فَبَايَعَ رَسُولَ اللَّهِ ﷺ عَلَى الْهِجْرَةِ وَلاَ يَشْعُرُ النَّبِيُّ ﷺ أَنَّهُ عَبْدٌ فَجَاءَ سَيِّدُهُ فَقَالَ النَّبِيُّ ﷺ بِعْنِيهِ فَاشْتَرَاهُ بِعَبْدَيْنِ أَسْوَدَيْنِ وَلَمْ يُبَايِعْ أَحَدًا بَعْدُ حَتَّى يَسْأَلَهُ أَعَبْدٌ هُوَ قَالَ وَفِي الْبَابِ عَنِ ابْنِ عَبَّاسٍ