[simonh]

In practice I don't think it's useful to think about it in terms of liability. That $100 bill is a share in the net wealth of the nation. This is why when a country prints a lot if its currency to fund government spending, the relative value of that currency goes down. There are more 'shares' in the economy now, so each one represents a smaller slice of the pie than it did before. So if there is any liability, it's distributed across everyone else holding that currency.

Of course the value of anything is only relative to what anyone is prepared to pay for it, it's a matter of sentiment, so the value of an economy can go up or down depending on perceived risks, liabilities or opportunities and thus is reflected in the value of the currency compared to other assets or currencies.