It was narrated that Jabir said: A slave came and gave his pledge to the Prophet pledging to emIgrate,. and the Prophet P.B.U.H did not realize that he was a slave. Then his master came looking for him, and the Prophet said: 'Sell him to me: and he bought him in exchange for two black slaves. Then after that he did not accept the pledge from anyone until he had asked whether he was a slave:' (Sahih)