Reference for Bava Metzia 5:9
לֹא יֹאמַר אָדָם לַחֲבֵרוֹ, הַלְוֵינִי כוֹר חִטִּין וַאֲנִי אֶתֵּן לְךָ לַגֹּרֶן. אֲבָל אוֹמֵר לוֹ, הַלְוֵינִי עַד שֶׁיָּבֹא בְנִי, אוֹ עַד שֶׁאֶמְצָא מַפְתֵּחַ. וְהִלֵּל אוֹסֵר. וְכֵן הָיָה הִלֵּל אוֹמֵר, לֹא תַלְוֶה אִשָּׁה כִּכָּר לַחֲבֶרְתָּהּ עַד שֶׁתַּעֲשֶׂנּוּ דָמִים, שֶׁמָּא יוֹקִירוּ חִטִּים, וְנִמְצְאוּ בָאוֹת לִידֵי רִבִּית:
One may not say to his neighbor: "Lend me a kor of wheat, and I will give it back to you at harvest time,"; but he may say to him: "Lend me until my son comes," or "until I find the key." [For since he has (produce), it is legitimate, the sages having decreed (against it) only when he has none. And even if he has only one sa'ah, he may borrow several kor upon it. For with each one we say, this will be its replacement, it not being acquired by the lender, and the borrower being permitted to sell it or eat it. And when he borrows each one, he does so licitly.] And thus did Hillel say: A woman may not lend a loaf to her neighbor until she "converts" it to money, lest wheat become dearer and they come to ribith (transgression). [The halachah is not in accordance with Hillel, but with the sages, who say that one may lend and repay "plain" (i.e., without "conversion")].