halacha - What happens if you borrowed money and the currency became worthless?

In Israel in the late 70s early 80s the currency (Lira) became worthless. Suppose one day the American dollar would become worthless, and one owes another $1,000,000- would the borrower just pay back 1 million worthless dollars? Or is there a responsibility to pay back the value (purchasing power perhaps) of the money when borrowed? (sources would be appreciated)