Terumot, Chapter Two, Mishnah Two
This mishnah is the opposite of yesterdays mishnah. Here we learn what happens if a person tries to give terumah from impure produce for pure produce. Obviously, this is much more problematic, because the impure terumah is far less useful to the kohen.
1) They may not give terumah from impure [produce] for that which is pure.
2) If he did give:
a) If unwittingly, the terumah is valid;
b) If intentionally he has done nothing.
3) So too, if a Levite had [unclean] tithe [from which terumah] had not been given, and he gave terumah from this, if unwittingly, the terumah is valid, if intentionally he has done nothing.
4) Rabbi Judah says: if he knew of it at the outset, even if done in error, he has done nothing.
Section one: One should not give terumah from impure produce on behalf of pure produce.
Section two: If, despite the fact that this is prohibited, the person did so anyway, if he did it without knowing what he was doing, the terumah is valid. However, if he did it intentionally, the terumah is not terumah and he must go back and take terumah out again.
Section three: As I stated in the introduction, a Levite must give terumah from the tithes that he receives. The Levite in our section has a pile of impure tithes and he tries to say that this tithe will be terumah on behalf of other tithes that he has received. The same rules apply here as applied in the case of regular terumah in section two. When it comes to the terumah taken from tithes, one also cannot give impure terumah to exempt pure terumah. However, Albeck notes that in the case of terumah from tithes, the opposite is not trueone can give from pure produce for impure produce.
Section four: Rabbi Judah holds that if at some point he knew that the pile of tithes was impure, then even if he later forgot and thought that he was giving terumah from pure produce, his terumah does not count. The fact that he once knew means that he can never again be considered as having given it accidentally.