Avodah Zarah, Chapter Five, Mishnah Seven
The first two sections of this mishnah discuss situations in which a Jew is considered as selling yen nesekh, which therefore means that the proceeds from the sale are prohibited.
The second two sections deal with situations in which a Jew is selling his wine to a non-Jew.
1) If a non-Jew sent to Jewish craftsmen a cask of yen nesekh as their wages, they are allowed to say to him, give us its value in money;
(a) But after [the wine] has come into their possession [the exchange] is prohibited.
2) If [a Jew] sells his wine to a non-Jew, should he have set the price before he measured it out, the purchase-money is permitted;
(a) But should he have measured it out before he set the price, the purchase-money is prohibited.
3) If [a Jew] took a funnel and measured [wine] into a non-Jews flask and then measured some into a Jews flask, should a drop of the [first] wine have remained [in the funnel], then [the wine measured into the second flask] is prohibited.
4) If he poured from [his own] vessel into [a non-Jews] vessel, [the wine in the vessel] from which he poured is permitted and [the wine in the vessel] into which he poured is prohibited.
Section one: If a non-Jew who has hired a Jew to do some work for him, pays him with yen nesekh and actually gives the Jew the wine, the Jew cannot exchange the wine back for money. Once the wine comes into his possession it is actually his, and exchanging it with the non-Jew would in essence be selling yen nesekh, which is forbidden. If the Jew were to send back the shipment of wine before it reaches him then he may accept in its place money. Since in this case the non-Jew has never paid back his debt, it is not considered as if the Jew is selling his own yen nesekh.
Section two: If a Jew sells his wine to a non-Jew, he must make sure that he is not receiving money for what is now yen nesekh. If he measures out the wine to the non-Jew and then sets a price, he is actually receiving payment for selling yen nesekh, since the wine becomes yen nesekh as soon as it is in the non-Jews possession. Therefore, he must set a price before he gives over the wine. Note that once the price is set he need not receive the money, for the debt of the non-Jew is already fixed.
Section three: In this section a Jew pours his wine into the container of a non-Jew using a funnel. The liquid that remains in the funnel when the Jew pours the wine into the non-Jews flask has the same status as the wine in the non-Jews flask itself. The fact that the wine in the flask is yen nesekh, means that the wine in the funnel is as well. This is because the funnel goes into the non-Jews container and may take back some of the yen nesekh. If even a drop of wine should remain in the funnel and the Jew should then pour more wine into the funnel and give it to a Jew, all of the wine has become contaminated as yen nesekh and it is all forbidden.
Section four: This section teaches a distinguishing principle from the previous section. When one pours from one vessel into another, and the poured liquid never touches both vessels at the same time, there is no contamination from the lower vessel into the higher vessel. While the bottom liquid might become itself contaminated (if it is poured into a non-Jews flask), the top liquid remains permitted.
We should note, that although these issues sound somewhat trivial, this issue, that of contamination flowing upwards through a poured liquid, was an issue of major content between different groups of Jews during the time of the Second Temple. In a famous document called The Halakhic Letter found in Qumran (The Dead Sea Scrolls the author of the document complains that his opponents (probably the Pharisees) claim that the poured liquid is pure. This complaint is also mentioned as a complaint of the Sadducees against the Pharisees in Mishnah Yadayim 4:7. We see that our mishnah takes the Pharisaic point of view.