Musannaf Ibn Abi Shayba 13:646

Abu Bakr narrated to us, he said: Abu Dawud al-Tayalisi narrated to us, from Hisham al-Dastawa'i, from Yahya ibn Abi Kathir, that Umar sent a servant of his, or a slave of his, with a sa' of wheat to buy for him with it a sa' of barley, and he rebuked him if they added more for him, that he should accept the addition.