How is the exchange rate of foreign currency calculated for paying Zakat on deposits—is it based on the black market rate or the banks' rate, and is it the exchange rate at the time of conversion or the average exchange rate throughout the year?

Whoever wishes to pay Zakat on paper currency with another paper currency should give what is more beneficial to the poor if the exchange rates differ. The prevailing exchange rate at the time of giving is what matters, not the average price throughout the year. It is not permissible to deposit money in interest-based banks.