Avodah Zarah 61 - Contract to Make Kosher Wine
Consider the following: a Jew contracts with an idolater to press the idolater's grapes into kosher wine and sell it to Jews, with the understanding that he would pay the idolater for his grapes only after selling the wine. Until then, he stores it in the idolater’s premises. To make sure that the idolater does not make a libation of this wine, the Jew stores it in a storage room open into the public domain, so that anyone inside can be observed. This helps if the town is inhabited by both Jews and idolaters, and a Jew is likely to look inside at any time. Otherwise, the Jew must appoint a guardian. The guardian does not need to be there at all time, but can go out and come in intermittently.
Furthermore, the idolater must write a receipt stating, "I have received money from you as a payment for the produce," so that the wine is in full control of the Jew and he can remove it any time that he wants. The obligation is thus converted to a regular loan, and there is no lien on the wine. With this, the wine is permitted.
Art: Gustav Vermehren - Farmhouse Interior with an Open Door