Terumot, Chapter Two, Mishnah One
One must give terumah from both pure and impure produce. The priest will be able to use the pure terumah for its general purpose and the impure produce he will have to burn but he can enjoy the produce while it is being burned. For instance, if he is given impure oil, he can burn it and use it to light his candles. If he is given impure wheat, he could burn it in his oven.
Our mishnah teaches that one cannot give terumah from pure produce in order to exempt impure produce, even though this might seem to be beneficial to the priest.
1) They may not give terumah from pure [produce] for impure [produce], but if they did give, the terumah is terumah.
2) In truth they said: If a cake of pressed figs had become partly defiled, one may give terumah from the clean part for that part which had become defiled.
a) The same applies to a bunch of vegetables, or a stack of grain.
3) If there were two cakes [of figs], two bunches [of vegetables], two stacks [of grain], one pure and one impure, one should not give terumah from one for the other.
4) Rabbi Eliezer says: one can give terumah from that which is pure for that which is impure.
Section one: As I stated in the introduction, one cannot give terumah from pure produce in order to exempt impure produce. If, however, he does so, his terumah is still valid.
Section two: The phrase, In truth they said, usually introduces a halakhah that deviates from the previous general rule. If one has a cake of pressed figs only part of which was impure, or a bunch of vegetables or a stack of grain, only part of which was impure, one can separate terumah from the pure part in order to exempt the entire cake, bunch or stack. Because it is all one pile or one group of produce, this is not considered taking from one pile/stack of pure produce on behalf of a different impure pile/stack.
Section three: However, as stated in section one above, if the cakes, bunches of vegetables or stacks of grain were separate, then one should not give terumah from the pure one on behalf of the impure one.
Section four: Rabbi Eliezer disagrees with the rules above and holds that one can give terumah from the pure for the impure.