Musannaf Ibn Abi Shayba 13:3214

Abu Bakr narrated to us, he said: Al-Dahhak ibn Makhlad narrated to us, from Ash'ath, who said: It was among the fundamental principles of Al-Hasan’s opinion that he saw no harm in [exchanging] one [measure] of sound copper for two [measures] of broken copper. And he was asked about [exchanging] one knife for two knives, and he saw no harm in it.