What is the nisab for Zakat on banknotes, and is its nisab estimated based on the nisab of gold or silver?

The zakat threshold (nisab) for banknotes, such as the dollar, is equivalent to the value of 20 mithqals of gold or 140 mithqals of silver at the time zakat becomes due. This is calculated in the manner most beneficial to the poor. Since the value of the silver nisab is currently lower than that of the gold nisab, the nisab is estimated based on silver, which is approximately 595 grams. When the money reaches this nisab and a full lunar year (hawl) has passed, zakat is paid at the rate of one-quarter of one-tenth (25 out of every thousand).