Ruling 2306
If a person gives something to someone so that he may take a greater amount in another city, and if the item is gold, silver, wheat, or barley which can be weighed or measured, it is usury and unlawful. However, if the party taking the extra amount gives or does something in return, there is no problem. If banknotes are given on loan, it is not permitted to take back more even though the amount of debt has decreased due to high prices. However, if the value of the loan goes down too much due to inflation and the like, it is an obligatory precaution to reach a settlement (ṣulḥ). If banknotes are sold for banknotes, and the sale is an immediate exchange (naqd)[1] or credit (nasīʾah) transaction but the money is in two currencies, such as pounds sterling and dollars, then there is no problem with any extra received. However, if it is a credit sale and the money is in one currency only, then receiving an extra amount is problematic (maḥall al‑ishkāl) [i.e. based on obligatory precaution, one must not receive an extra amount].[2]
-Ayatullah Sistani, Practical Laws of Islam, Loan (Qard)
In an immediate exchange transaction, there is no lapse of time between the buyer paying for the item and receiving it.