Musannaf Ibn Abi Shayba 13:3442

Abu Bakr narrated to us, he said: Waki’ narrated to us, he said: I heard Sufyan saying: If a man came to a money changer with a dinar and exchanged it with him for ten dirhams, and he (the money changer) took possession of the dinar, but the money changer did not have a dirham (at that moment). He said: If he obtained them for him before they parted, then the sale is permissible, because each one of them is a price for the other. And if it were merchandise, the sale would be void.