Sahih al-Bukhari Book 34, Hadith 94
Allah's Messenger (ﷺ) forbade Najsh.
A man decided that a slave of his would be manumitted after his death and later on he was in need of money, so the Prophet (ﷺ) took the slave and said, "Who will buy this slave from me?" Nu'aim bin `Abdullah bought him for such and such price and the Prophet (ﷺ) gave him the slave.
Allah's Messenger (ﷺ) forbade the sale called 'Habal-al-Habala which was a kind of sale practiced in the Pre- Islamic Period of ignorance. One would pay the price of a she-camel which was not born yet would be born by the immediate offspring of an extant she-camel.