From: Rabbi Moshe Sofer
To: Unknown recipient
A widow claiming her kesubah presented a single witness who testified that her dying husband forgave her the oath. The Chasam Sofer rules that since the mechilah was private, it constitutes only a weak migo-based claim, and following the Rema and Mordechai, a supporting witness cannot exempt from the oath in such a case; all authorities would agree the widow must still swear.
Source: Shu"t Chasam Sofer CM §69