I kind of enjoyed the twist. and the plot as a whole.
But, in line with the show's apparent general inability to do basic research, currency does not work like that. The Denar would not drop in value to zero just because the nation was about to convert to euros. The majority of Denars like the majority of all money would be electronic entries, and after the conversion those electronic entries would simply be converted to euros either at a fixed rate or according to a formula - in either case negotiated before admission. Actual paper currency would be exchangeable at the same rate or by the same formula for a generous period of time after the euro became the official currency. Depending on whether joining the euro was expected to be beneficial or harmful to the economy, the Denar would undergo a small increase or decrease in value once admission became likely. But it would not drop by a great deal prior to admission, and if it dropped slightly the increase upon failure of admission would be equally small. Normally such a deal would not be made in the first place unless the conventional wisdom was that it would be beneficial, meaning the Denar would rise slightly once it became clear that the referendum was likely to pass, and would fall slightly after an unexpected failure.