Commentary for Avodah Zarah 120:3

If a non-Jew is seen standing next to a vat of wine we need to know if the non-Jew touched the wine. The mishnah teaches that if the non-Jew had loaned money to the Jew then the wine is prohibited. In such a case, the non-Jew has a lien on the Jew’s wine and might at some point say to the Jew, give me the wine and I will forgive you the loan. Since he has this financial connection to the Jew, and in some sense he has ownership over the wine, he will feel free to take some of the wine for himself. Therefore we must assume that he has come into contact with the wine. If, however, he has not loaned money to the Jew, then we do not assume that he had come into contact with the wine, and it is permitted.