Date: 8 Elul 5571
From: Rabbi Moshe Sofer
To: Unknown recipient
Rabbi Mordechai Benet rules that since standard promissory notes include a clause requiring repayment in cash, borrowers must repay loans in current full-value currency after the 1811 Austrian devaluation of Bancozettel to one-fifth face value. Even when the due date preceded the devaluation, the stipulation applies at the time of actual payment. Where a royal decree permits payment in old currency but does not benefit the king, it may not qualify as binding dina d'malchusa dina, and in that doubt the muchzak prevails.
Source: Shu"t Chasam Sofer CM §65