(23) باب جَوَازِ بَيْعِ الْحَيَوَانِ بِالْحَيَوَانِ مِنْ جِنْسِهِ مُتَفَاضِلاً
Jabir (Allah be pleased with him) reported:
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)
Reference: Sahih Muslim 1602
In-book reference: Book 22, Hadith 152
USC-MSA web (English) reference(deprecated numbering scheme): Book 10, Hadith 3901