The article discusses a problem - frequent in the international trade relations - concerning a set-off of claims (receivables) which are denominated in different currencies. The author uses a case-study technique to outline the main issues concerning the determination of law applicable to set-off in cross-border cases. The application of Article 17 of the Rome I regulation (concerning contractual obligations) is advocated also in situations when one of the claims involved has a non-contractual origin In the second part of the article the permissibility of setting off of claims denominated in different currencies is analyzed under Polish substantive law. The author supports a liberal position in this respect and argues that such set off should be, by and large, permitted. Some observations are also made regarding the exchange rates that ought to be used to calculate the value of claims subject to the set-off mechanism.