From: Rabbi Moshe Sofer
To: Rabbi Yaakov Segal
The Chasam Sofer rules that where a wife is mentally incompetent and cannot yet receive a get, the husband may not discharge his kesubah obligation with depreciated paper currency, since the payment date is indeterminate and the currency may further decline. Instead, he must provide security guaranteeing the wife's maintenance during her illness and payment of 400 Zehuvim in whatever currency is current upon her recovery, with a floor of no less than the Torah-mandated 200 silver Zwanziger.
Source: Shu"t Chasam Sofer EH II §165