How is Zakat paid on 500 Euros that have reached the Nisab and a full year has passed on them, given their continuously changing price in Egyptian Pounds, and is only one Zakat due, or more, since more than one year has passed on the amount?

Zakat (definition) is obligatory on banknotes if their value reaches the lower of the two thresholds (Nisab (definition)) for gold or silver, which is equivalent to 85 grams of gold or 595 grams of silver. The amount of 500 Euros reaches the Nisab, and Zakat becomes obligatory on it after one year of ownership. The amount of Zakat is 2.5% of the total sum, which is 12.5 Euros, and it should be disbursed from the same currency or its equivalent in another currency at the time it becomes due. It is not permissible to delay the disbursement of Zakat after a year has passed over the money. Zakat is only required for one year. If the second year is completed and the money is still in possession, Zakat for the second year must be disbursed.