Reference for Bava Metzia 5:10
אוֹמֵר אָדָם לַחֲבֵרוֹ, נַכֵּשׁ עִמִּי וַאֲנַכֵּשׁ עִמָּךְ, עֲדֹר עִמִּי וְאֶעְדֹּר עִמָּךְ, וְלֹא יֹאמַר לוֹ נַכֵּשׁ עִמִּי וְאֶעְדֹּר עִמָּךְ, עֲדֹר עִמִּי וַאֲנַכֵּשׁ עִמָּךְ. כָּל יְמֵי גָרִיד, אֶחָד. כָּל יְמֵי רְבִיעָה, אֶחָד. לֹא יֹאמַר לוֹ חֲרשׁ עִמִּי בַּגָּרִיד וַאֲנִי אֶחֱרשׁ עִמְּךָ בָּרְבִיעָה. רַבָּן גַּמְלִיאֵל אוֹמֵר, יֵשׁ רִבִּית מֻקְדֶּמֶת וְיֵשׁ רִבִּית מְאֻחֶרֶת. כֵּיצַד. נָתַן עֵינָיו לִלְווֹת הֵימֶנּוּ, וְהָיָה מְשַׁלֵּחַ לוֹ וְאוֹמֵר בִּשְׁבִיל שֶׁתַּלְוֵנִי, זוֹ הִיא רִבִּית מֻקְדֶּמֶת. לָוָה הֵימֶנּוּ וְהֶחֱזִיר לוֹ אֶת מְעוֹתָיו, וְהָיָה מְשַׁלֵּחַ לוֹ וְאָמַר בִּשְׁבִיל מְעוֹתֶיךָ שֶׁהָיוּ בְטֵלוֹת אֶצְלִי, זוֹ הִיא רִבִּית מְאֻחֶרֶת. רַבִּי שִׁמְעוֹן אוֹמֵר, יֵשׁ רִבִּית דְּבָרִים, לֹא יֹאמַר לוֹ, דַּע כִּי בָא אִישׁ פְּלוֹנִי מִמָּקוֹם פְּלוֹנִי:
One may say to his neighbor: "Weed with me [today] and I will weed with you [tomorrow]; dig with me, and I will dig with you." But he may not say to him: "Weed with me, and I will dig with you; dig with me, and I will weed with you. [Sometimes one (activity) is more difficult than the other, and agar natar ("reward for waiting") obtains.] All the days of the dry season are one, and all the days of the rainy season are one. [And we are not concerned about one day being longer than the other. And it is permitted to tell one: "Dig with me this day of the dry season, and I will dig with you a different day of the dry season." And so, with the rainy season.] He may not say to him: "Plow with me in the dry season, and I will plow with you in the rainy season." [For field work is more difficult in the rainy season.] R. Gamliel says: There is antecedent ribith and supervening ribith. How so? If he thought to borrow from him and he sent him a gift, thinking: "So that you lend me," this is antecedent ribith. If he borrowed from him and returned his money and sent him a gift, saying: "For the money that was 'idle' with me," this is supervening ribith. R. Shimon says: There is ribith of words: He (the borrower) should not say to him: "Know that that man is coming from that place."