I wouldn't say it's an edge case; it eventually will happen to any perpetuity.

When a currency disappears on a planned basis (like the Deutschmark) the government issues a recall of the currency in banks at a set exchange rate: e.g. for the Deutschmark it was set by law at 1.95583 DM = 1 euro (source: https://en.wikipedia.org/wiki/Deutsche_Mark)

When a currency disappears entirely (like with Zimbabwe), there's a much more chaotic basis as it switches to separate sovereign currencies, which often have different inflation rates baked in. As this can be much more unexpected than a planned currency transition, the market often doesn't incorporate the expected changes in real interest rates, and often the exchange rate needs to keep floating for a while to account for ongoing inflation. More info on Zimbabwe's abandonment of their currency: https://en.wikipedia.org/wiki/Zimbabwean_dollar#Abandonment_...

Seems like the easiest way would be to tie the payment to the cost of a basket of goods at the time of the issuing of the debt or at the cessation of the currency, whichever paid less (assuming you are the person paying) and then converting that to dollars at the date of the cessation of the original currency and then paying that amount.