Commentary for Avodah Zarah 118:12

The baraita clearly allows grapes carried by non-Jews to the winepress. Even if some liquid comes out of the grapes, the grapes do not become yayin nesekh.

There are manuscripts that read "Rav Ashi said to them." And this also appears in the commentary of the Ravad.......

R. Ashi seems to think that merely by touching the wine the gentile has not yet prohibited it. The problem will come about if he stirs the wine—meaning moves his hand about. R. Ashi seems to think that we need to be concerned lest he libate the wine. So R. Ashi instructs his students to hold the non-Jew’s hand in place. Then the Jews should pour the contents of the cask out, thereby maintaining them as permitted.

If the non-Jew knows that by libating the Jew’s wine he will render it unsaleable, the Jew may recover the cost of the wine. This is not considered “deriving benefit” but merely protecting himself from a loss. The non-Jew is looked at as a damager who must compensate the damaged party for his loss.

R. Ashi cites his source, a ruling by R. Judah b. Bava and R. Judah b. Betera. They both hold that a non-Jew cannot just libate a Jew’s wine and thereby make it prohibited. The Tosafot say that Rav Ashi is not actually ruling like R. Judah b. Bava and R. Judah b. Batera. These rabbis allow the wine to be sold to anyone. From here R. Ashi deduces that while the other rabbis would not let the wine be sold to anyone, they would allow (or mandate) the non-Jew to compensate the Jew for his loss.
There are stories in the Middle Ages of Christians actually doing this—touching Jewish wine intentionally to cause the Jews a loss. The rabbis responded by allowing Jews to even drink such wine. They did not want this to be a way for Christians to cause massive losses to Jews.

According to this version of the story, R. Papa prohibited only the wine that was on the same level as the spout. The rest of the barrel, what was above it and below it, was permitted. We don’t consider all of the wine to be connected.