What should I do if the other bank has not paid after the Iranian letter of credit is negotiated because of the exchange rate difference?

Under normal circumstances, if the issuing bank does not mention any discrepancies within 5 working days after receiving the documents, it will default to a clean ticket and the issuing bank will perform the payment responsibility. The letter of credit is independent of the contract and is not affected by external factors. In this case, you can apply to an international arbitration institution for arbitration, or even go through judicial channels.

PS: I admire your courage to open a letter of credit with Iran. Apart from Iran being sanctioned by the United States, few banks in Iran have a good reputation. When talking about the contract, you should communicate this problem with Kunlun Bank. Only Kunlun Bank in China can do L/C business with Iran, and others are afraid of joint liability for receiving US sanctions.