The local currency in Serbia is the Serbian dinar (RSD). While cards are widely accepted at most businesses, cash payments in euros or US dollars are not permitted. However, in many bars and restaurants, the waiting staff may exchange currency for you at a slightly less favorable rate.
Currency exchange offices are readily available and typically offer guaranteed exchange rates. You should not encounter any fees or commissions for these services. To assist with your search for currency exchange, you can use the term "menjacnica."