Related for Menachot 132:18

They may give tithes from produce from a Jew for produce [bought] from a Gentile, from produce [bought] from a Gentile for produce from a Jew, from produce [bought] from a Jew for produce [bought] from Samaritans, and from produce [bought] from Samaritans for produce [bought] from [other] Samaritans. Rabbi Eliezer prohibits [tithing] from produce [bought] from Samaritans for produce [bought] from [other] Samaritans.

We may tithe [certainly untithed produce] from a Jew on behalf of [produce bought from] a Samaritan [which is also certainly untithed]; and [certainly untithed produce] from a Samaritan on behalf of [certainly untithed produce bought from] a Jew (Dem. 5:9); and [following the same formula] from a Jew on behalf of a non-Jew; and of a non-Jew on behalf of a Jew, and of everyone on behalf of everyone else, the word of Rabbi Meir. [But] Rabbi Yehudah, and Rabbi Yosei, and Rabbi Shimon say, we may tithe [certainly untithed produce] from a Jew on behalf of [certainly untithed produce from] another Jew; from a Samaritan on behalf of another Samaritan, and from a Gentile on behalf of another Gentile; but not from a Jew on behalf of a Samaritan, and not from a Samaritan on behalf of a Jew, and not from a Gentile on behalf of a Jew, and not from a Jew on behalf of a Gentile. Said Rabbi Shimon Shezuri, it so happened that my untithed produce got mixed up [with tithed produce], and I came and aksed Rabbi Tarfon [what to do]. He told me, go and buy produce from the market and tithe it on behalf of them (i.e., the mixed-up produce). Rabbi Eliezer says (following Erfurt Manuscript), with respect to produce from a Samaritan, just as they (i.e., mixing them in with one's own produce) convert a Jew's produce into Demai according to the law of the majority, [so too] we do not take Terumah and tithes from one on the other, and thus we tithe a Samaritan's produce according to the laws of Demai, and we do not tithe one on behalf of the other (i.e., a Samaritan's produce on behalf of a Jew's produce, or vice-versa).