We may not separate terumah from tamei [produce] on behalf of tahor [produce]. If one separated terumah [thus], [if he did so] inadvertently, his terumah is terumah; but [if he did so] intentionally, he has done nothing. Similarly, a Levi who had maaser [rishon which is] tevel, [and] he was continuously separating from it [tithes for other produce], [if he does so] inadvertently, what he did is done; [but if he does so] intentionally, he has done nothing. Rabbi Yehudah says: If he knew of it initially, even though his [designation] was inadvertent, he has done nothing.