Related על גיטין 46:16

A Gentile that separates the terumah of his friends (חבריו not חברים per the GR"A), or of a Jew, even with permission, his terumah is not [valid] terumah (Ter. 1:1). It so happened in Pinah (alt. "in Pigah" =בפיגא), that a certain Jew said to a Gentile, "Separate the terumah of my threshing floor." And he separated the terumah, and [subseqently] the terumah fell back onto the threshing floor [becoming mixed with the rest of the produce]. And this matter came before Rabban Gamaliel (alt., "Shimon ben Gamaliel") and he said, "Since the Gentile separated the terumah, it is not [valid] terumah," [and thus the resulting mixture is not forbidden]. Rabbi Yitzhak says, a Gentile that separates the terumah of a Jew, and the owners [of the produce] remain near him [throughout the process], his terumah is [valid] terumah.

A Gentile that [separates] terumah -- his terumah is valid terumah (but see Ter. 1:1). In what case does this apply? [In the case of] a threshing floor, he separated terumah and gave it to a Kohen, [or separated] first tithes and gave them to a Levite [or separated] poor person's tithes and gave them to a poor person. [If his] produce is properly tithed [but] he brings his produce into his house, his produce is [considered] spoiled. An Israelite who is suspected of having brought his produce into his house, his produce [is considered] spoiled. A Samaritan is like a Gentile, the words of Rebbi. Rabban Shimon ben Gamaliel says, a Samaritan is like an Israelite.