Implementation of monetary and currency policies and regulating the currency market in particular, are considered as the Central Bank's duties. The banking system plays an important role in development and fmancing, especially in the international commerce and in thi s arena, establ ishing order and security on the one hand, and respecting private ownership and facilitat ion of trade should be taken into account altogether; Nowadays (particu larly after the Currency Crisis of 20 12), claiming the currency exchange rate's difference h as turned into one of the most prevalent banking lawsuits and this matter is significant in this regard, since in the mentioned currency crisis of that year, the currency exchange rate's difference subject to claim was by the amount that the trade was depri ved of i ts economic justification and even in several cases, entailed the loss of applicant as well. By examining the various presumptions of pre-payment, interim payment and non-payment, we reach this general conclusion that with respect to the currency sale declarat ion, which is filled out at the time of endorsing the documents by the name of applicant, the sale of currency at the mentioned t i me is fixed and its Rial equivalent at this date, would be the criterion for the final settlement between the applicant and the issuing bank and the undertaking letter on currency fluctuat ions would also be pertinent to the changes in the currency, from the time of opening the l etter of credit to the time of endorsement and transaction of documents and claiming the currency exchange rate's difference before the fixed and final day of settlement breaches the laws, regulations and other directi ves of the Central Bank.