From: Rabbi Moshe Sofer
To: Unknown recipient
A follow-up letter sharing the Chasam Sofer's personal study notes on Tosafos in Sotah 3b, clarifying why a special verse is needed for a single witness to establish a sotah's prohibition despite the general principle of eid echad ne'eman b'issurin. He concludes that Tosafos must rely on hafka'as kiddushin rather than the Ran's mechanism of shavya nafshei, and since hafka'as kiddushin is only rabbinic, Abaye's verse remains necessary at the Biblical level.
Source: Shu"t Chasam Sofer EH II §107