משנה על בכורים 2:1

They may not give terumah from pure [produce] for impure [produce], but if they did give, the terumah is terumah. 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. The same applies to a bunch of vegetables, or a stack of grain. 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. Rabbi Eliezer says: one can give terumah from that which is pure for that which is impure.