Bava Metzia 72A

אית ליה זכיה מדרבנן. הכא נמי לא שנא.

nevertheless, he has the power, by rabbinic law, to acquire an item by means of an act of acquisition performed by another? Here, also, it is not different; the Jew can perform an act of acquisition on behalf of the non-Jew even though there is no agency for non-Jews.

ולא היא: ישראל אתי לכלל שליחות, נכרי לא אתי לכלל שליחות.

The Talmud rejects this comparison: But that is not so. A minor Jew will eventually reach the stage of eligibility for agency, but a non-Jew will not reach eligibility for agency. Consequently, the rabbis did not establish the power for non-Jews to acquire an item by means of an act of acquisition performed by a Jew.

תנו רבנן: ישראל שלוה מעות מן הנכרי ברבית, וזקפן עליו במלוה, ונתגייר. אם קודם שנתגייר זקפן עליו במלוה ⁠– גובה את הקרן וגובה את הרבית. ואם לאחר שנתגייר זקפן עליו במלוה ⁠– גובה את הקרן ואינו גובה את הרבית.

§ A baraita states: In the case of a Jew who borrowed money with interest from a non-Jew, and the non-Jew lender established the interest as a loan for the Jewish borrower, i.e., he added the amount of interest to the principal and consolidated it into a single debt, and then the non-Jew converted, the halakha depends on the circumstances. If the non-Jew established it as a loan for him before he converted, he may collect the principal from the borrower and he may also collect the interest. Since it had already been consolidated into a single debt, it is as though he already collected the interest while he was still a non-Jew. But if he established it as a loan for him after he converted, he may collect the principal but may not collect the interest, as it still had the status of interest when he became a Jew.