Русско-англо-русский юридический словарь

It would seem that if party wishes to be more sure that the currency of a contract should remain in euros, regardless of the lex monetae principle, it should include words which make clear, for example, that the currency of the contract will remain in euros regardless of whether the country of payment remains within the European Monetary Union or not. (Practical Law)

A currency union (also known as monetary union) is where two or more states share the same currency, though without there necessarily having any further integration such as an Economic and Monetary Union, which has, in addition, a customs union and a single market. (Wikipedia)

monetary union Two or more countries with a single currency, or different currencies having a fixed mutual exchange rate monitored and controlled by one central bank (or several central banks with closely coordinated monetary policies). (BusinessDictionary)

An economic and monetary union is a type of trade bloc which is composed of an economic union (common market and customs union) with a monetary union. It is to be distinguished from a mere monetary union (e.g. the Latin Monetary Union in the 19th century), which does not involve a common market. This is the fifth stage of economic integration. EMU is established through a currency-related trade pact. (Wikipedia)