Musannaf Ibn Abi Shayba 13:3187

Abu Bakr narrated to us, he said: Ibn ‘Uyaynah narrated to us, from ‘Amr, who said: I asked ‘Ata’: “I buy [something] for a thousand dirhams, and I say before the contract is concluded: ‘I will make it [equivalent to] one hundred dinars.’” He said: “There is no harm in it.”