Is it permissible to agree to waive the increase or decrease in the exchange of metal currency, whether before or after the exchange?

Prohibited contracts are not made permissible by mutual consent between the contracting parties, for the reason for the prohibition is not the lack of consent. Furthermore, most of what the Qur’an and Sunnah have forbidden in transactions aims at establishing justice and prohibiting injustice, such as consuming wealth unlawfully, usury (riba), gambling (maysir), gharar sales, and others.

The general reasons for the invalidity of sales are: the prohibition of the sold item itself, usury (riba), gharar (excessive uncertainty), and conditions that lead to one or a combination of these matters. Therefore, usury (riba) does not become permissible even if both parties consent.