That statement is true. The amount owed does not change....

That statement is true. The amount owed does not change. However, when money devalues by way of inflation then suddenly the people of the country are compensating by dealing in higher volumes of money to compensate for it's plumetting value. Say for instance it is a government debt... Then tax revenue increases because the government is now collecting more money, which has lost value. Afterall, inflation is the rising cost of goods and or services.