'Uthman bin 'Affan reported Allah's Messenger (ﷺ) as saying: Do not sell a dinar for two dinars and one dirham for two dirhams.
Explanation & Benefits
Shaykh Maulana Abdul Aziz Alvi
Hadith Commentary: Benefits and Issues: Nowadays, internationally, paper currency is considered as cash just like dinar and dirham. With it, anything can be purchased just as with dinar and dirham. Therefore, its ruling will also be the same as that of dinar and dirham, and zakat will be taken from it just as from dinar and dirham. If someone possesses currency equivalent to the value of fifty-two and a half tola of silver, then after a year passes, two and a half percent zakat must be paid on it.
Source: Tuhfat al-Muslim: Commentary on Sahih Muslim, Page: 4058