From: Rabbi Moshe Sofer
To: Unknown recipient
The Chasam Sofer argues that the requirement of techilaso v'sofo b'kashrus for inter-witness relatedness does not apply in monetary matters, since each witness individually retains some testimonial function, though it does apply in kiddushin where a single witness is wholly ineffective. Regarding ona'ah, he limits R' Avigdor's principle that the doubtful party bears the burden of proof to internal doubts about the substance of a transaction, not external factors like market price, grounding the Rema's ruling instead in chazakah.
Source: Shu"t Chasam Sofer CM §37