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)
Bava Kamma 97a/b, last line. Rav says you must pay back value, Shmuel says you can pay back even that currency and say "go spend it where it works". And even Shmuel only meant when you could get to such a place. I would presume that a with regards defunct currency that wont work anywhere, even Shmuel would agree that you must pay back value. The halachah (Choshen Mishpat 74:7) is like Shmuel.