How is the Nisab calculated, and is Zakat obligatory on money in hard currency (Euros) given the difference in its price between the black market and banks? Is Zakat calculated based on the black market price or the bank price, especially since the hard currency is saved for the purpose of studying or receiving medical treatment abroad?

The default is to pay zakat (definition) for each currency separately, such as paying zakat for the local currency in dinars and for hard currency in euros. However, it is permissible to pay all of it in dinars if that is more beneficial for the poor, while calculating the hard currency at its highest value. This is to ensure maximum benefit for the poor and to discharge the zakat payer's obligation fully.