Musannaf Ibn Abi Shayba 13:3116

Abu Bakr narrated to us, he said: Umar ibn Ayyub narrated to us, from Ja'far ibn Burqan, he said: I asked al-Zuhri about a man who buys *fulus* (copper coins) with a dirham; is it considered *sarf* (currency exchange)? He said: Yes, so do not depart from him until you have taken possession of it in full.