What is the ruling on selling paper riyals for metal riyals with a difference in amount?

Question : The questioner says: Honorable Shaykh, you mentioned in a fatwa of yours regarding the difference between the metal and paper Saudi riyal, and you stated that it is permissible for one to take ten paper riyals and give nine metal ones. We ask for clarification of this for the students of knowledge, as much debate has arisen among them regarding this fatwa. May Allah reward you with good.

Answer : I say, yes, I issued this fatwa: if a person gives nine metal dirhams for ten paper dirhams, there is no harm, but on the condition that the exchange takes place immediately without delay. This is because these dirhams and these papers are in the position of small coins (fulus). The scholars mentioned in the chapter on riba (usury or interest) and currency exchange that there is no riba in fulus; there is no riba al-fadl (usury of excess in a like-for-like exchange) in them. And if there is no riba al-fadl, it is permissible to make a difference in amount between them. Furthermore, from the perspective of their nature, their type is different: this is metal and this is paper. The fact that one metal riyal is considered equal to one paper riyal is merely a declaration by the state, and it is not based on supply and demand. Supply and demand vary; the value of the paper may increase, or the value of the riyal may increase. I recall that when the paper riyal was first issued, people preferred it over the silver riyal; they chose to take the paper riyal. People preferred the paper riyal over the silver riyal. Now, ask those who deal in currency exchange what the silver riyal is worth?
We find that it is worth seven paper riyals, and it may increase or decrease. So, the declaration by the state does not mean that this is the value according to supply and demand. People need these metal coins for telephone calls. And perhaps they cannot find someone to give them ten for ten.
So, if they take nine for ten, there is no harm, but on the condition of immediate exchange, because when currency is sold for another currency, immediate exchange is obligatory. From here we move to what some people are doing now in trading currency. The question about this is frequent: Is it permissible to trade in currency, meaning that I buy currency with another currency and wait for profit? The answer to this is that it is permissible and there is no harm, but on the condition of immediate exchange, meaning the taking possession of both