סימן סה סעיף יא
אָסוּר לְהַלְווֹת לְאֶחָד סְאָה תְבוּאָה שֶׁיַּחֲזִיר לוֹ אַחַר כָּךְ סְאָה תְבוּאָה, אֲפִלּוּ מִין בְּמִינוֹ, כִּי שֶׁמָּא תִּתְיַקֵּר בֵּינְתַיִם הַתְּבוּאָה וְנִמְצָא זֶה מַחֲזִיר יוֹתֵר מִמַּה שֶּׁלָּוָה. אַךְ יַעֲשֶׂנוּ דָּמִים, שֶׁאִם תִּתְיַקֵּר הַתְּבוּאָה לֹא יִתֵּן לוֹ רַק הַדָּמִים. וְאִם יֵשׁ לַלֹּוֶה אֲפִלּוּ רַק מְעַט מִמִּין זֶה, מֻתָּר לִלְווֹת אֲפִלּוּ כַּמָּה כּוֹרִין. וְכֵן אִם יֵשׁ לְאוֹתוֹ מִין שַׁעַר קָבוּעַ בַּשׁוּק, מֻתָּר לִלְווֹת, אַף עַל פִּי שֶׁאֵין לַלֹּוֶה כְּלוּם מִזֶּה הַמִּין. וְכָל זֶה, בְּמִין בְּמִינוֹ. אֲבָל מִין בְּשֶׁאֵינוֹ מִינוֹ, כְּגוֹן לְהַלְווֹת סְאָה חִטִּין בִּסְאָה דֹּחַן, אָסוּר בְּכָל עִנְיָן, אַף עַל פִּי שֶׁהֵן בְּשַׁעַר אֶחָד וְיֵשׁ לוֹ דֹּחַן. וּבְדָבָר קָטָן שֶׁאֵין דֶּרֶךְ לְהַקְפִּיד בְּיוּקְרָא וְזוֹלָא, מֻתָּר בְּכָל עִנְיָן. וְלָכֵן מֻתֶּרֶת אִשָּׁה לְהַלְווֹת כִּכַּר לֶחֶם לַחֲבֶרְתָּהּ (קס"ב).
Preface to Paragraph 11
It was previously mentioned in the preface to this chapter that ribis deoraisa applies where the lender and borrower, at the time of the loan, agree to a set amount of interest to be repaid. If the rate of interest is not set, or if at the time of the loan it is uncertain whether interest will actually be paid, then this constitutes ribis derabanan.
The following paragraph discusses one aspect of ribis derabanan.
Unlike money whose value, halachically, remains constant, the value of goods and services fluctuate. This creates a problem when lending goods and services, because at the time of payment, albeit the same quantity is returned, nevertheless, the value of the said goods may have increased thereby giving the lender a profit on his loan. This kind of ribis is known as סָאָה בְּסָאָה, (sa'ah be'sa'ah) i.e. "a bushel for a bushel." The derivation of this name stems from the situation where a bushel of grain is lent with the stipulation that a bushel be repaid. Since the repaid bushel may increase in value, this constitutes ribis derabanan.
An exception to this prohibition: If the borrower of the goods has the identical goods in his possession at the time of the loan which can potentially be used as payment at the time of the loan.
You may not lend someone a sa'ah of grain [with the stipulation] that he repay a sa'ah of grain even if they are both of the same species, because there is the possibility that, in time, the price of the grain will increase and, consequently, he will be repaying [value] greater than what he borrowed. Rather, a money valuation should be set on the grain, so that if the price of the grain increases, he will pay you only the amount of money [at which the grain was originally valued]. However, if the borrower owns even a small amount of that kind [of grain], he may borrow many korim.12Kor (pl. korim): a large measure of grain. Similarly, if that particular [grain] has a set price in the marketplace, he may borrow [sa'ah besa'ah], even though the borrower does not possess any [grain] of that kind. All of the above applies where the same species of grain [are involved], but where two different species [are involved]; for instance, lending a sa'ah of wheat for a sa'ah of millet is prohibited under any circumstances, even if their prices are the same and [the borrower] posesses millet.13There is a great likelihood that the prices of the two grains will not stay the same, creating the possibility that the repayment will be more than the loan. When [the loan is] an insignificant amount, where it is unusual to be concerned about higher or lower prices, [sa'ah besa'ah] it is permitted in all cases. Therefore, a woman is permitted to lend a loaf of bread to her friend.
תנא דבי אליהו כל השונה הלכות בכל יום מובטח לו שהוא בן העולם הבא שנאמר הליכות עולם לו אל תקרי הליכות אלא הלכות (נדה ע"ג.)