Ibrahim (may Allah have mercy on him) said: There is no harm in an advance sale (salaf) in copper coins (fulus). Sa'id al-Qaddah (may Allah have mercy on him) also said: There is no harm in an advance sale (salaf) in copper coins.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10527