Commentary for Avodah Zarah 61:7

R. Judah b. Batera classifies three different types of wine. The first is wine that has actually been libated. Such wine is considered to be a by-product of idol worship. It is forbidden to derive any benefit from this wine. And it causes impurity. Wine that belongs to non-Jews is not considered to have been involved in idolatry. But it is still forbidden to derive benefit from it, and of course to drink it. Its defiling properties are lesser—it only defiles food and drink.
If the wine belongs to a Jew, but the non-Jew was holding on to the wine, then the Jew may derive benefit from the wine, but he may not drink the wine.
Note that there seem to be two issues combined here. The first is contact with items used in idolatrous ritual. This would apply either to wine that had actually been libated, or to wine that might have been libated. The second issue is preventing contact with non-Jews. Wine simply owned by a non-Jew does not need to be treated as an idolatrous product. But if the goal of this halakhah is to prevent fraternization, then prohibiting any contact would make some sense.

The question at hand is whether one holding on to a deposit belonging to another is considered to be the deposit’s owner (even a temporary one). The earlier baraita seemed to say no—that is why it is not prohibited to derive benefit from the wine. But this mishnah (from Sheviit) says that the produce is considered to belong to the non-Jew and therefore it is not subject to the laws of tithes or the Sabbatical year.
The resolution is that in the case of the wine, he assigned it a separate corner, and thus did not touch it.

If the non-Jew set aside a special corner for the wine, then why shouldn’t the Jew be allowed to drink it? The Talmud bolsters the question with a story about R. Yohanan coming to Parud.
There he asks R. Tanhum to a halakhah in the name of his teacher, Bar Kappara. The halakhah that is quoted contradicts what was said above. As an aside, R. Yohanan is happy to hear the teaching and considers him a proper student of Bar Kappara. In any case, we now have contradictory tannaitic sources.

R. Zera solves the difficulty by ascribing the baraita that prohibits to the rabbis and the baraita that permits to R. Eliezer. However, R. Eliezer permits only if the Jew holds the key or the seal, such that it would be difficulty for the non-Jew to break into the wine.

According to R. Elazar, if one wants to guard, for instance, one’s food, such that it remains kosher, one seal around the package is enough (I know—on the airplane they put two seals). The exception is wine—wine must be guarded by a double seal. R. Yohanan says that one seal is always sufficient.

This is an unusual use of the term “and they are not arguing” because in the end R. Elazar and R. Yohanan do argue. R. Elazar holds like the rabbis—a seal is not sufficient to protect wine. R. Yohanan holds like R. Eliezer—a seal does guard the wine.

This is a more stringent version of the dispute between R. Elazar and R. Yohanan. According to this version, all agree that one seal is not sufficient to ensure that the wine is not touched. The argument is over whether two seals are needed.

En-Kushi, Borkata and Zagdar are Samaritan cities while Birat-Sirika, Kefar-Parshai and Kefar Shalem are non-Jewish cities. Originally they prohibited the wine in the Samaritan cities due to their proximity to non-Jewish cities. But then they relaxed this law and allowed Jews to drink Samaritan wine if the cask had been closed up.

The change happened when the rabbis realized (or simply decided) that Samaritans did generally ensure that non-Jews should not come into contact with their closed casks of wine. They just were not so strict, and thus if a cask was opened, Jews could not be sure that non-Jews had not touched it.

This source describes a Jew who sends a substance with either a Samaritan or a non-Jew. Rashi explains that with brine the fear is that the non-Jew may switch it with his own brine that has non-kosher fish in it. Murias is also made of fish, but Rashi explains that it might have wine in it. In any case, if the Jew can tell that the wine, brine or murias has not been opened then he may drink it. But simply closing the cask is not sufficient.