Commentary for Avodah Zarah 121:11

R. Ashi says that in the mishnah the cask was not thrown into the vat from afar. It was rolled in and the non-Jew still had contact with it when it touched the wine. This, if it were done not in anger, would make the wine nesekh, just as it would make the wine impure if done by a zav. Thus there is a similarity between the rules governing the case of yayin nesekh and the case of the zav.

If a Jew were to prepare wine belonging to a non-Jew and then leave it on the non-Jew’s property we need to know whether or not the non-Jew had contact with the wine and thereby made it into yen nesekh. If the house was open to the public domain and there were both Jews and non-Jews living in the city, the wine is permitted. The reason is that the non-Jew will fear that if he touches the wine a Jew passing by might see him and tell the other Jews, in which case they won’t buy the wine from him. This non-Jew from the outset wanted to sell to non-Jews therefore he won’t perform any act that might cause him to lose his ability to sell the wine. However, if there are only non-Jews in the city, the non-Jew does not fear that they will see him and report him to the Jews. Since in this case he is not afraid to touch the wine, the wine must be guarded to make sure that it doesn’t become yayin nesekh. If it is not guarded the law is strict and it is forbidden.
This guardian need not sit and guard the wine 24 hours a day. It is sufficient for him to come in and out occasionally. As long as the non-Jew does not know when he will come in and out, the non-Jew will be too afraid to touch the wine for fear that he will be caught. This is similar to the way that Jewish kashruth supervisors work today. They are not present in restaurants at all times. It is enough for the restaurant owner to know that they might show up at any time for him to be afraid to break the rules of kashruth. The words of Rabbi Shimon ben Elazar are not easy to explain. The Talmud will try to interpret them.

In this case the non-Jew has already written out a receipt that he has received the money for the wine. Even if he preemptively wrote the receipt before he received the money, in this case the non-Jew will assumedly not touch the wine. If the Jew really has paid the money then it simply belongs to the Jew, and the non-Jew will not touch wine that doesn’t belong to him. If the Jew has not really paid the money, the non-Jew will not touch it for if he does, the Jew will not pay him.

However, if the non-Jew demonstrates that he doesn’t consider the wine to really belong to the Jew until he pays the money, then we cannot assume that he has not touched it. Since the non-Jew still considers himself the owner, he may allow himself to take some of the wine, even though the Jew made it in order to subsequently pay for the wine. The mishnah relates that this case actually happened in Bet Shean.

Even if the city’s residents are only non-Jews, there are still Jewish peddlers roaming around. These peddlers would inhibit the non-Jew from touching the Jew’s wine. So why is the wine not permitted without a guardian in all case.

If the city is locked up and Jewish merchants cannot come in, then the non-Jew will be able to drink the Jew’s wine with impunity. This would mean that it is prohibited. By implication, if the city is not locked, then the Jew’s wine is permitted.

If the wine were in the public domain, we can assume that the non-Jew would not drink it. If the house has a window, then people can see in. If there is a rubbish-heap then people could stand on it and peer into the person’s house (I know this is strange). They might also be able to see in when they climb up a date-palm.

If the top of the date-palm is cut off then there would seem to be no reason for anyone to climb to the top. For this reason, one rabbi permits the wine. But another rabbi prohibits, thinking that the Jew might climb the tree to look for a lost animal.

The question we need to ask is whether the non-Jew will enter the house and take a sip from the Jew’s wine. If there is another Jew living in the courtyard, then he will not do so, out of fear that the Jew will report him to his fellow tribesman. If the Jew has the key and seal, then he will not be able to enter without breaking the lock and seal. But if there are no other Jews in the courtyard, and the non-Jewish landlord has the key and seal, then we need to be concerned. After all, he thinks he will not be caught.