The breif decription about foreign exchange in Iran :

The Iranian currency is Rial,which can be exchanged  at the airports,some branches of banks and the exchange shops.Ferdowsi square is one of the most important places in tehran due to economic point of huge presence of exchange shops there.Most of them,better say nearly all of these exchange shops has SANA code which is confirmation from Central Bank of Iran to work in this field specially for doing international exchanges .If you are a tourist and you need to do your cash be exchanged its is necessary to provide your ID card or your passport as an identity document.The rates change often but there are many sources to check the rates daily.Two currencies which are used mostly in Iranian market are US DOLLAR & EURO,other currencies are not as much popular as these two.We can give the third place to CHINA's YUAN which is mostly used because of Iran and china economic & trading exchanges.Some of Iranian banks has branches which allows the Iranian and foreign customers to open and foreign currency account.These account are for international trading,credit refrence point to opening LC or investments  uses also has a limit & ,minimum amount for opening this accountmprovided by each bank.As and example if a company wants to check a businessman credit level it asks to provide the print of balance and interactions of his foreign accounts ,it show's the incomings & outgoings of that company or businessman ,so the reliability.The rate of exchange used in governmental companies or semi-governmental (a semi-government is and institute which is both public and private at the same time and it rules are mixed and follows both government and private part bureaucracy) are different from the rate in market .
But the final tip to you, try to find the best source to check the daily rate based on institute or person you work with.
Here are some links that might be helpful:
Central Bank of IRAN: https://www.cbi.ir/default_en.aspx
Sana Rate under control of Central Bank of IRAN:https://fxmarketrate.cbi.ir
Ports & Maritime organisation rate for calculating THC: http://saoi.ir/Persian_Site/curency/curencys.aspx