Rashi in d.h. harei yesh ba heter achilah says that Terumah cannot be redeemed and concludes "Ve'haomer ken Rosho who"
Do you have any idea what prompted Rashi to make this unusual comment?
Good question. This certainly seems to be an odd statement coming from the pen of Rashi.
The KAPOS TEMARIM asks this question. He adds that at worst, someone who does not know an explicit Halachah (that Terumah cannot be redeemed) is an Am ha'Aretz, but not a Rasha.
He answers that Rashi means as follows. It cannot be that since ("Migu") one can redeem an Esrog of Terumah Tehorah (and eat it himself), one may use it for the Mitzvah of Esrog even without redeeming it, because there is no such thing as redeeming Terumah. Normally, one who thinks that Terumah can be redeemed is an Am ha'Aretz. Here, though, this person not only thinks that it is possible to redeem Terumah, but he also thinks that it is permitted to do so on Yom Tov. Although it is possible to be uneducated and think that Terumah has Pidyon, one who goes so far as to say that it can be redeemed on Yom Tov -- thereby contravening the explicit declaration of the Rabanan who prohibited doing any act of Pidyon on Yom Tov, is called a Rasha, for everyone knows that there is no Pidyon on Yom Tov, and yet this person purposely claims that there is in order to permit using an Esrog of Terumah Tehorah.
This is the answer of the Kapos Temarim. The question, though, still requires further elucidation.