Hakim asked (the Prophet): Messenger of Allah, a man comes to me and wants me to sell him something which is not in my possession. Should I buy it for him from the market? He replied: Do not sell what you do not possess.
Sunan Abi Dawud Book 24, Hadith 88
← Previous Hadith On his father's authority, said that his grandfather told that the Messenger of Allah (ﷺ) forbade the type of transactions in which earnest money was paid.

Malik said: This means, as we think--Allah better knows-that a man buys a slave or hires an animal, and he says: I give you a dinar on condition that if I give up the transaction or hire, what I gave you is yours.

Sunan Abi Dawud Book 24, Hadith 87 Next Hadith → On his father's authority, said that his grandfather 'Abd Allah b. 'Amr reported the Messenger of Allah (ﷺ) as saying: The proviso of a loan combined with a sale is not allowable, nor two conditions relating to one transaction, nor profit arising from something which is not in one's charge, nor selling what is not in your possession.

Sunan Abi Dawud Book 24, Hadith 89