From: Rabbi Moshe Sofer
To: Rabbi Moshe
The Chasam Sofer rules that paper banknotes carry no one-fifth tolerance for fluctuation, so any change in value is legally significant. When currency depreciates after a debtor delays past the due date, the loss falls on the delaying party. For self-imposed obligations such as dowry commitments this applies in practice, while for actual loans it is ruled in theory only due to ribis concerns.
Source: Shu"t Chasam Sofer CM §74