Shevuot 36B

אלא איפוך ⁠– אפילו באיסורא לית ליה.

Rather, nevertheless, reverse the attribution of the opinions in the Mishnah and say that R' Meir deems the witness exempt from liability for an oath of testimony, as even in ritual matters, R' Meir does not accept the principle: From a negative statement you can infer a positive statement.

מתקיף ליה רבינא: ובאיסורא לית ליה?! אלא מעתה, שתויי יין ופרועי ראש ⁠– דבמיתה; הכי נמי דלית ליה לרבי מאיר?! והתנן: אלו שבמיתה ⁠– שתויי יין ופרועי ראש!

Ravina objects to this: And in ritual matters, does R' Meir not accept the principle? But if that is so, with regard to the halakha that priests who perform the Temple service while intoxicated with wine, and priests who perform the Temple service with overgrown hair on their head, who, based on this principle, are liable to receive death at the hand of Heaven, so too does R' Meir not accept this principle? But didn’t we learn in a baraita: And these are they who are liable to receive death at the hand of Heaven: Priests who perform the Temple service while intoxicated with wine and priests who perform the Temple service with overgrown hair on their head, and R' Meir does not disagree with that ruling?

אלא לעולם תיפוך; כי לית ליה ⁠– בממונא, באיסורא אית ליה; ושאני סוטה דאיסורא דאית ביה ממונא הוא.

Rather, actually reverse the attribution of the opinions in the Mishnah and say that R' Meir deems the witness exempt from liability for an oath of testimony. When R' Meir does not accept that principle, it is only in cases involving monetary matters; but in cases involving ritual matters, he accepts the principle. That is the reason he does not dispute the halakha in the baraita with regard to a priest intoxicated with wine or with overgrown hair on his head. And the reason that he does not accept the principle: From a negative statement you can infer a positive statement, in the case of sota is that sota is different because it is a ritual matter in which there are ramifications involving monetary matters, i.e., payment of the marriage contract. The same is true with regard to an oath of testimony in the Mishnah; although it is a ritual matter, it is a ritual matter with ramifications involving monetary matters.