Khalid bin Mahran al-Hadha' خالد بن مهران الحذاء · Commercial Transactions (Kitab Al-Buyu)

The Messenger of Allah (ﷺ) said: Gold is to be paid for with gold, raw and coined, silver with silver, raw and coined (in equal weight), wheat with wheat in equal measure, barley with barley in equal measure, dates with dates in equal measure, salt by salt with equal measure; if anyone gives more or asks more, he has dealt in usury. But there is no harm in selling gold for silver and silver (for gold), in unequal weight, payment…