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 curren

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.