Does booking a ticket for my brother for a certain amount, on condition that he repays me the same amount in a different currency upon his return, constitute usury (riba), especially if the exchange rate of the second currency increases?

The amount for which the travel ticket was purchased is established as a debt owed by the debtor brother, in the currency that was paid. It is not permissible to agree to pay the debt in another currency in advance. However, it is permissible to agree on that on the day of repayment according to the exchange rate at that time, because deferred currency exchange is riba nasi’a (interest due to delay). If the debtor wishes to reward his brother, he may do so after paying the debt or its value at the exchange rate on the day of repayment, for the Prophet, peace and blessings be upon him, said: "Indeed, the best of people are those who are best in settling their debts."