Tosefta על תרומות 3:9

A Jewish Priest [can] eat with a Samaritan Priest when [the food is] made in [a state of] impurity, but when [it is] made in [a state of] purity, behold, he does not eat it with him, because he feeds him the meat of an unblemished first-born [calf], and has him drink wine from a [ritually] pure fourth[-year vineyard]. [See Deut. 15:22 and Lev. 19:24, and discussions at Bekh. 5:2 (also Bekh. 32b:10) re: first-born calf, and Ter. 3:9 and Y. Peah VII.5.2 re: fourth-year vineyard.]

A Gentile that separated a first-born calf, a donkey, or challah, we inform him [that he is not permitted to do this] so that he does not become liable. They put [the first-born calf] to work and they shear it [to dissuade him from dedicating it], but afterwards, [if he persists,] we accept it from him. And the challah can be eaten by non-Kohanim. One who removes terumah from inside his house, it is treated like untithed produce and terumah, the words of Rebbi. Rabban Shimon ben Gamaliel says, we do not treat it as anything but great terumah. If he removes first tithe from inside his house, it is treated like untithed produce and first tithe, the words of Rebbi. Rabban Shimon ben Gamaliel says, we do not treat it as anything but first tithe. If he removes second tithe from inside his house, and he said, "It is redeemed!" he has not said anything. If he said, "It is saved!" his words are permanent. And a Samaritan is like a Gentile, the words of Rebbi. Rabban Shimon ben Gamaliel says, a Samaritan is like an Israelite.

A Samaritan that separated terumah and gave it to a [Samaritan] priest (see Lieberman), his terumah is [valid] terumah. Rabbi Shimon ben Elazar says, it is [valid] terumah but he goes back and takes terumah a second time [and gives to a Jewish priest, i.e., a Kohen]. They said to him, "What is the difference between this case and a Kohen that took terumah and brought it before his livestock [which is permitted]." He said to them, "The difference is this -- that this one separates terumah with sanctity, and that one does not separate terumah with sanctity."