(Exodus 22:24) "If you lend money to My people." Rabbi Ishmael says: Every "if" in the Torah denotes something optional, except for "if you lend money," which is an obligation. Or is it perhaps optional? Scripture teaches, "You shall surely open your hand to him" [literally: lend, lend to him] (Deuteronomy 15:8), indicating an obligation and not an optional matter. "If you lend money" teaches that you lend him money for money, but you do not lend him produce for produce. You lend him money for money, but you do not lend him money to be repaid in produce, nor produce to be repaid in money [to avoid hidden interest through price changes].
When You Lend Money to My People It Is a Duty Not a Choice
Yalkut Shimoni on Torah 350:1
(שמות כב כד) אִם כֶּסֶף תַּלְוֶה אֶת עַמִּי, רַבִּי יִשְׁמָעֵאל אוֹמֵר, כָּל "אִם" שֶׁבַּתּוֹרָה, רְשׁוּת, אִם כֶּסֶף תַּלְוֶה, חוֹבָה. אוֹ אֵינוֹ אֶלָּא רְשׁוּת, תַּלְמוּד לוֹמַר (דברים טו, ח) "הַעֲבֵט תַּעֲבִיטֶנּוּ", חוֹבָה וְלֹא רְשׁוּת. אִם כֶּסֶף תַּלְוֶה, כֶּסֶף בְּכֶסֶף אַתָּה מַלְוֵהוּ, וְאִי אַתָּה מַלְוֵהוּ פֵּירוֹת בְּפֵירוֹת. כֶּסֶף בְּכֶסֶף אַתָּה מַלְוֵהוּ, וְאֵין אַתָּה מַלְוֵהוּ כֶּסֶף בְּפֵירוֹת וּפֵירוֹת בְּכָסֶף.