Musannaf Ibn Abi Shayba 13:2256

Abu Bakr narrated to us, he said: Abda bin Sulayman narrated to us, from Sa'id, from Qatada, from Sa'id bin al-Musayyib and al-Hasan, that they used to see no harm in exchanging white dirhams for black dirhams, as long as it was not a condition.