Is it permissible to agree on a currency exchange rate over the phone and then exchange it hand-to-hand on a later day through representatives?

"Mawada 'ala al-sarf" (fixing the exchange rate with future delivery and receipt) is a matter of scholarly disagreement. The more accurate opinion is that it is permissible if the agreement is non-binding on both parties, provided that the exchange takes place hand-to-hand. However, a binding agreement between both parties is prohibited because it falls under "bay' al-kali' bi al-kali'" (selling debt for debt).