Is it considered usury if the intention is to pay an additional amount when exchanging currency after the session has ended, despite the exchange having occurred at a lower rate?

The buyer's acceptance of the exchange rate, and the completion of the قبض (seizure/possession) and the تفرق (separation), concludes the transaction. The additional two thousand (currency units) that were not seized/received in the session are considered a gift or a present, and are not part of the original exchange contract.