"Muggle" refers to an individual who isn't born with magical powers and is incapable of using them.
In a country where both muggles and wizards have co-existed amicably for several centuries, technology is similar to that of the modern day. Since leaving the gold standard, people have to believe that the paper money (notes) in their hands must contain value even through it isn't linked to gold anymore. The wizards, on the other hand, have a special fiat system similar to paper money, but they use scrolls instead. Similarly, the wizards must agree that each piece of scroll must be worth a value, and that the scrolls cannot be tampered with magic. In order to establish a foreign exchange for both muggles and wizards to do trade, mutual trust is important. Without tampering with free and abusing magic, how could foreign exchange work for the muggles and wizards? (All wizards are familiar with DnD.)
Given that muggle gold acts as a commodity used to strengthen paper money, what would the wizards use to strengthen scrolls?