יורה דעה קע״אYoreh De'ah 171

Se’if 1א׳

יִשְׂרָאֵל שֶׁלָּוָה מָעוֹת מֵהָעוֹבֵד כּוֹכָבִים בְּרִבִּית וּזְקָפָן עָלָיו בְּמִלְוֶה, וְנִתְגַּיֵּר, אִם עַד שֶׁלֹּא נִתְגַּיֵּר זְקָפָן עָלָיו בְּמִלְוֶה, גּוֹבֶה קֶרֶן וְרִבִּית. וְאִם מִשֶּׁנִּתְגַּיֵּר זְקָפָן עָלָיו בְּמִלְוֶה, גּוֹבֶה הַקֶּרֶן וְלֹא הָרִבִּית. אֲבָל עוֹבֵד כּוֹכָבִים שֶׁלָּוָה מִיִּשְׂרָאֵל בְּרִבִּית וְזָקַף עָלָיו אֶת הָרִבִּית בְּמִלְוֶה, אַף עַל פִּי שֶׁזְּקָפָן עָלָיו אַחַר שֶׁנִּתְגַּיֵּר, גּוֹבֶה אֶת הַקֶּרֶן וְאֶת הָרִבִּית, שֶׁלֹּא יֹאמְרוּ: בִּשְׁבִיל מְעוֹתָיו נִתְגַּיֵּר זֶה, וְגוֹבֶה הַיִּשְׂרָאֵל מִמֶּנּוּ אַחַר שֶׁנִּתְגַּיֵּר כָּל מְעוֹת הָרִבִּית שֶׁנִּתְחַיֵּב בָּהֶם כְּשֶׁהָיָה עוֹבֵד כּוֹכָבִים.

An Israelite who borrowed money from an idolater (oved kokhavim — the standing term for a gentile) at interest, and he set it upon him as a loan, and he converted: if he set it upon him as a loan before he converted, he collects principal and interest. And if he set it upon him as a loan after he converted, he collects the principal and not the interest. But an idolater who borrowed from an Israelite at interest, and the Israelite set the interest upon him as a loan — even though he set it upon him after he converted, he collects the principal and the interest, so that they shall not say: it was for his money that this man converted. And the Israelite collects from him, after he converted, all the interest money in which he became liable while he was an idolater.