American sanctions forbid dollar transactions with Iranian participants. To get round the problem, European banks effect transactions on behalf of Iranian banks. Debits and credits are transferred to these unnamed accounts via the European bank's usual American correspondents in New York. With only the account number to show on the documentation, no one in the United States is any the wiser as to who the recipients are. Without this mechanism, Iran would be cut off from the international trading community.
Iranian banks pay extra commission.
They have to suffer. If sanctions are withdrawn, this will change. Iranian banks will be able to deal directly with corresponding banks in New York and one of the primary impetuses for Iranians to trade from Dubai will disappear. What is more, with the Iranian Government able to relax foreign exchange restrictions, the dhow trade that has for so long supported the smaller traders of Dubai will become a fraction of what it is today. Official Iranian exporters have always been at a disadvantage in relation to their smaller cousins.