What serves as a substitute for gold and silver, such as paper currency, is subject to obligatory alms (zakah).

The Shaykh : Second: What serves as a substitute for gold and silver is paper currency. If a person possesses paper currency that reaches the minimum threshold (nisab), it is obligatory for him to pay zakah on it; otherwise, he does not. Does paper currency in our current time equal silver or is it less? Who says it equals it? If so, prepare for us, and we will come to you tomorrow with a hundred notes and you will give us a hundred silver riyals? Does it equal silver now? Do these existing notes equal silver? No, it is less. And what I heard is that the silver riyal is worth ten paper riyals. This is what we heard. If that is the case, how much is the nisab in paper?
The student: ... fifty-six. Give us the knowledge. All right.

The Shaykh : If the value of the silver riyal is ten paper riyals, the nisab in paper is five hundred and sixty riyals. All right. And if the value of the silver riyal were five paper riyals, how much would the nisab be? Two hundred and eighty. All right. And if the riyal were equal to the riyal, it would be fifty-six notes as the nisab. So, by what do we consider the value of the paper? By silver. Take the original nisab of fifty-six in silver and its equivalent in paper. And ask the money changers about the value. All right. How much is this? Gold and silver, and the second is paper currency.