Siman 441
Presented here for study, not as a practical ruling. The Shulchan Aruch rules by Sephardi custom, the Rema's glosses by Ashkenazi custom, and the Kitzur follows Ashkenazi custom. For a practical question, ask your rabbi.
א עַכּוּ"ם שֶׁהִלְוָה לְיִשְׂרָאֵל עַל חֲמֵצוֹ וּמִשְׁכְּנוֹ בְּיָדוֹ, וְאָמַר לֵיהּ מֵעַכְשָׁו יְהֵא שֶׁלְּךָ אִם לֹא אֶפְרַע לְךָ לִזְמַן פְּלוֹנִי, וְהִגִּיעַ הַזְּמַן וְלֹא פְּרָעוֹ, מֻתָּר אֲפִלּוּ לֹא הִגִּיעַ הַזְּמַן שֶׁקָּבַע עַד אַחַר הַפֶּסַח; דְּמִשֶּׁהִגִּיעַ הַזְּמַן וְלֹא פְּרָעוֹ, נִקְנָה לוֹ לְמַפְרֵעַ וְהָוֵי לֵהּ חֲמֵצוֹ שֶׁל עַכּוּ"ם.
1 A non-Jew who lent to an Israelite on his Chametz and took it as a pledge in his hand, and said to him: "From now it shall be yours if I do not repay you by a certain time," and the time arrived and he did not repay him, it is permitted even if the time he set did not arrive until after Pesach. Since when the time arrived and he did not repay him, it is acquired by him retroactively, and thus it is considered to be the Chametz of the non-Jew.
ב יִשְׂרָאֵל שֶׁהִלְוָה לְעַכּוּ"ם עַל חֲמֵצוֹ, אִם מִשְׁכְּנוֹ בְּיָדוֹ וְאָמַר לֵיהּ מֵעַכְשָׁו, וְהִגִּיעַ הַזְּמַן אֲפִלּוּ אַחַר הַפֶּסַח וְלֹא פְּרָעוֹ, אֲפִלּוּ אֵין אַחֲרָיוּתוֹ עָלָיו אָסוּר, מִשּׁוּם דְּקָנֵי לֵיהּ לְמַפְרֵעַ. אֲבָל אִם לֹא אָמַר לֵיהּ מֵעַכְשָׁו, וְאֵין אַחֲרָיוּתוֹ עָלָיו, מֻתָּר.
2 An Israelite who lent to a non-Jew on his Chametz, if he took it as a pledge in his hand and said to him: "from now," and the time arrived even after Pesach and he did not repay him, even if the responsibility for it does not fall upon him, it is forbidden, because he acquires it retroactively. But if he did not say to him: "from now," and the responsibility for it does not fall upon him, it is permitted: