International payment through POS Terminal

Generally the card brands apply their own currency exchange rates. Usually they don't differ too much from the exchange rate established by the central bank. The card brand that uses the cheapest currency exchange rate is MasterCard, being the cheapest on the market. On the other hand the card brand that applies the highest exchange rate is Amex. In reference to your question, the entity that will decide on the currency exchange rate applied for a particular transaction is the issuing bank of the card and depending on the brand of the card it will be higher or lower.