Is it permissible, Islamically, to set the wages of expatriate workers lower than those of local workers, based on the value of their countries' currencies, even though they exert the same effort and working hours?

The determining factor for wages is their agreement in the contract. There is no injustice if the worker accepts a lower wage, as long as there is mutual consent. It is commendable for a person to be generous in their dealings. The Messenger of Allah, peace and blessings be upon him, prayed for mercy upon the generous man when he sold, bought, and sought repayment, because this indicates generosity, magnanimity, and the love of Allah. Therefore, generosity towards workers is righteousness, and there is no injustice as long as there is mutual consent.