Is it permissible to exchange an old Kufan Dinar and a Dirham for a Sham Dinar of equal weight, when the Kufan Dinar is generally considered to be of lower value?
Chapter on Riba (Usury) and Exchange (Sarf)
Al-Mughni
Book of Sales
Primary text
It is not permissible to execute this exchange directly. The transaction is only permitted if the lesser-valued Dinar is reduced in value, and the difference is made up by giving the equivalent amount in silver.