What is the exchange rate to be considered for paying Zakat on wealth in hard currency: is it the banks' rate or the parallel market rate? And what is the expiation due for delaying the payment of Zakat past its due date?

The foundational principle for the nisab (minimum threshold for zakat) of monetary wealth is gold or silver. If the currency reaches the nisab by itself, or when combined with other money or trade goods, and a full lunar year (hawl) has passed, then 2.5% of the same currency should be given to those who are eligible. It is permissible to exchange it for another currency, taking into consideration what is most beneficial for the poor. It is not permissible to delay zakat after it becomes due, except for a compelling excuse. Whoever delays it without an excuse must hasten to pay it and should repent and seek Allah's forgiveness.