The ruling on exchanging paper currency for metal currency with an increase

Question:
Some shops exchange metal currency for less than the paper currency. For example, ten paper riyals for nine metal riyals. What is the ruling on this practice? May Allah reward you with good.

Answer:
It appears to me that this is not permissible because it is selling a riyal for a riyal with an increase, which constitutes riba (usury or interest). When selling a riyal for a riyal, equality is required, just as the Prophet (peace and blessings be upon him) commanded equality in gold and silver, and in these currencies that serve as substitutes for gold and silver. Equality is obligatory in them. If one sells, for example, a metal riyal for a paper riyal, equality is required, and an increase is not permissible. The difference between one being metal and the other being paper has no effect; it carries no meaning. Because it is all Saudi riyal, it is a single type; it is all one type, the Saudi riyal, whether it is metal or paper. As long as it is one Saudi riyal, exchanging some of it for some of it requires equality, otherwise it becomes riba if it occurs.