From: Rabbi Moshe Sofer
To: Rabbi Aharon Fulda
The Chasam Sofer agrees with the Taz that one who mistakenly believed an action was permitted (omer mutar) is not penalized when he nullifies a prohibited substance ab initio. His novel reasoning is that the rationale for penalizing ordinary shogeg — deterring deliberate violators who feign inadvertence — does not apply to omer mutar, since once the law is made known the person can no longer plead ignorance. He flags the Rambam's stricter ruling in Hilchos Terumos as requiring further investigation, and closes with an excursus on Rashi's varying explanations of Kohanim she-piglu.
Source: Shu"t Chasam Sofer YD §88