As we have learned, whenever a non-Jew touches wine and has the opportunity to pour a libation to a pagan god, the wine becomes forbidden as stam yeinam – ordinary non-Jewish wine. The Mishnah on today’s daf (=page) discusses how one can sell wine to a non-Jew, since the wine will become stam yeinam as soon as it is poured into his container. The Mishnah teaches that if the non-Jewish buyer and the Jewish seller came to an agreement regarding the price of the wine before it was measured out into the non-Jew’s container, then the money is permitted. If, however, the wine was first measured out and only afterwards the price was discussed, then the Jew cannot make use of the money since the wine had already become stam yeinam before the sale took place.
The Gemara connects this ruling with a general discussion of how Jewish law relates to business dealings between Jews and non-Jews. Ameimar rules that a kinyan meshikhah – an act of ownership performed by pulling or drawing an object towards you – is effective when done by a non-Jew, even if no money was exchanged. Rav Ashi disagrees, arguing that only if money is exchanged will ownership change hands. As proof he brings a case similar to the one in our Mishnah, where Rav instructed wine-sellers to collect payment before selling wine to non-Jews, or at least arrange a loan so that the non-Jews will be obligated to pay a loan rather than pay for the wine. The explanation that Rav gave was that if the wine was still owned by the Jew when it was placed in the hands of the non-Jew, no benefit can be derived from it, since it is considered yayin nesekh – wine libated to the gods.