Tosefta על תרומות 2:2

The landowner has the right to set aside tithes of untithed produce in the amount of the terumah of tithes that are in the tithes. Rabbi Yosei says, [in the case of] the landowner that separates terumah from tithes, what he has done is done.

Rabbi Eliezer says, we [may] take terumah from the pure on behalf of the impure (Ter. 2:1). Said Rabbi Eliezer, it so happened the threshing floors caught fire in Kfar Signah, and they took terumah from the pure on behalf of the impure. They said to him, what proof is that? Rather, [we should conclude] that they took terumah "from them on behalf of them" (i.e., from pure produce on behalf of other pure produce). Rabbi Ilai (אלעאיי not אליעזר per Lieberman) said in the name of Rabbi Eliezer, they [may] take terumah from the pure on behalf of the impure, even [when it comes to produce that is in] liquids. How is this done? Whoever pickles olives in [a state of] impurity and seeks to take terumah from them in purity, he brings a funnel whose mouth is not wider than an egg, and places it on the mouth of an amphora, and brings the olives and puts them inside [the funnel] and takes terumah, and [in this way] he is able to take terumah from the pure on behalf of the impure or the "earmarked" (המוקף, see Tos. Kifshutah). They said to him, nothing is considered "fluid" but wine and oil (see Y. Chal. II.3.9). Rabbi Yosei says, he who takes terumah from the impure on behalf of the pure, whether inadvertent or intentional, his terumah is [valid] terumah (see Y. Ter. VI.1.6). Said Rabbi Yosei, why should there be any difference between this case (i.e., taking terumah from the impure on behalf of the pure) and the case of taking terumah from the bad [quality] on behalf of the good (which is valid terumah (Ter. 2:6 ))?

[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).