Tosefta zu Bamidbar 18:28

[In summary,] we [may] separate terumah and tithes from the impure on behalf of the impure, and from the the pure on behalf of the pure, and from the impure on behalf of the pure, but not from the pure on behalf of the impure (contrary to Rabbi Eliezer above (and see Bava Kamma 115b:14)). Rabbi Nehemiah says, we do not separate [terumah] from the impure on behalf of the impure, but only as to demai. They said to him, behold, it says (Num. 18:28), "... and [they] are to give from them the contribution of God to Aharon the priest" (i.e., the Torah does not distinguish between pure and impure in relation to terumah).