Reference for Bava Metzia 124:6

אמר רבה מתניתין בבא לחוב בדמיהן עסקינן

'Wherever by their law there is no exaction from the debtor to the creditor, there is by our law no restoration from the creditor to the debtor;' this refers to prepaid and postpaid interest.<span class="x" onmousemove="('comment',' V. infra 75b. Such interest is not actionable in Gentile law, and therefore, if paid, is not returnable by Jewish law. ');">⁶</span>

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