וְחֵי אָחִיךָ עִמָךְ... וְתַרְבִּית וְיָרֵאתָ מֵאֱלֹקֶיךָ... נֶשֶׁךְ וְתַרְבִּית אַל תִּקַּח מֵאִתּוֹ... בְּמַרְבִּית לֹא תִתֵּן לוֹ... בְּנֶשֶׁךְ לֹא תִתֵּן לוֹ... אֶת כַּסְפְּךָ לֹא תִתֵּן לוֹ בְּנֶשֶׁךְ... אֶת אָכְלְךָ בְּמַרְבִּית לֹא תִתֵּן לוֹ (כ"ה ל"ו-ל"ז)
Do not take from him interest and increase, and you shall fear Hashem; and let your brother live with you. Do not give him your money for interest, and do not give your food for increase.
The Gemara in Bava Basra (י ע"א) says as follows. א''ר יוחנן מאי דכתיב מלוה ה' חונן דל, אלמלא מקרא כתוב אי אפשר לאומרו, כביכול עבד לוה לאיש מלוה. - What is the meaning of that which is written, ‘One who lends to Hashem is the one who is gracious to the poor’? R’ Yochanan said: If not that the Passuk had stated this idea unequivocally, it would have been impossible for us to have ever uttered it. The Passuk is implying as follows. Because Hashem is responsible to sustain every being, therefore, one who gives charity to the poor, is considered to have lent that money to Hashem. In view of this, the notion that the Passuk states, that ‘a borrower is a servant [i.e. is beholden] to the lender’, can be applied to one who gives charity; for when one gives charity to the needy, Hashem is, so to speak, beholden to him, just as the borrower is beholden to the lender.
Similarly, every time one spends money in the observance of a Mitzvah, it is as if Hashem is the borrower of that money, and is, so to speak, beholden to reimburse the money. This idea is described in the Gemara Beitza (טז ע"א) which says as follows. כל מזונותיו של אדם קצובים לו מראש השנה ועד ראש השנה, חוץ מהוצאת שבתות והוצאת יו"ט והוצאת בניו לתלמוד תורה, שאם פחת פוחתין לו ואם הוסיף מוסיפין לו - All of a person’s income is fixed each year, from Rosh Hashana until Rosh Hashana, with the exception of expenditures for Shabbos, Yom Tov and the teaching of Torah to one’s children. For in regard to these expenditures, if one spends less, he receives less, and if he spends more he receives more. A person’s total income for the year is decided on Rosh Hashana, therefore one should be careful not to spent extravagantly, because the income allotted for his basic essentials will not be increased beyond the predetermined amount. However, the amount which is allotted to a person to fund his observance of Mitzvos, is not fixed in advance. Rather, the profitability of his ventures will increase according to how much he spends on the Mitzvos.
In view of the fact that all money spent in the performance of Mitzvos, whether Tzedakah or any other Mitzva, is reimbursed by Hashem in full, the following Midrash (ויק"ר ל א) is hard to understand. From the reward of one ‘taking’ you may infer what the reward is for another act of ‘taking’. The Jewish People were commanded in Egypt ‘to take a bundle of eizov and dip it into the blood that is in the basin, and to touch the lintel and the two doorposts with some of the blood that is in the basin’. Now, the value of that bundle of eizov was very little, yet it caused the Jewish People to inherit the booty which the Egyptians brought along with them into the Red Sea, as well as the spoils of the conquered Sichon and Og and of the conquered thirty-one kings in Canaan. If such was the reward for observing such an inexpensive Mitzvah, then in the case of the Mitzvah of lulav, which certainly costs a lot of money, all the more so will one be awarded with much riches for fulfilling the Mitzvah.
This idea that Hashem pays so much reward for each and every Mitzvah that one does is hard to understand, as we shall explain. Every time a Jew pays money in order to do a Mitzvah, that money is, so to speak, akin to a loan to Hashem, which Hashem guarantees to pay back. Thus, because Hashem pays back the loan, it would be forbidden for Him to pay back any additional reward, for that would be a transgression of paying interest.
The Tur (יו"ד סי' ק"ס) writes the following in regard to Torah scholars borrowing from each other with interest.
תלמידי חכמים מותרין להלוות זה לזה ברבית לפי שיודעין שהוא אסור ונותנין אותה במתנה גמורה ודוקא בדבר מועט שאדם רגיל ליתן לחבירו במתנה אבל ביתר מכן לא - Torah scholars are permitted to lend to one another with interest because they know that interest is forbidden and they wholeheartedly give each other the money as a gift. We view the exchanges of money between Torah scholars as gifts, rather than as a loan and payment.
We can explain that when one gives Tzedakah or spends money on any Mitzvah, he does so without the intention of ever getting paid back or ever getting rewarded, as the Sages say, הוו כעבדים המשמשים את הרב שלא על מנת לקבל פרס - Be like servants who serve their master not for the sake of receiving a reward. Therefore, Hashem is allowed to ‘gift’ the person with an extra measure as well, and it isn’t a transgression of the prohibition of interest.
(זרע שמשון פרשתנו אות ד)
