Tosefta for Peah 5:5
הַמַּחֲלִיף עִם הָעֲנִיִּים, בְּשֶׁלּוֹ פָּטוּר, וּבְשֶׁל עֲנִיִּים חַיָּב. שְׁנַיִם שֶׁקִּבְּלוּ אֶת הַשָּׂדֶה בַּאֲרִיסוּת, זֶה נוֹתֵן לָזֶה חֶלְקוֹ מַעְשַׂר עָנִי, וְזֶה נוֹתֵן לָזֶה חֶלְקוֹ מַעְשַׂר עָנִי. הַמְקַבֵּל שָׂדֶה לִקְצֹר, אָסוּר בְּלֶקֶט שִׁכְחָה וּפֵאָה וּמַעְשַׂר עָנִי. אָמַר רַבִּי יְהוּדָה, אֵימָתַי, בִּזְמַן שֶׁקִּבֵּל מִמֶּנּוּ לְמֶחֱצָה, לִשְׁלִישׁ וְלִרְבִיעַ. אֲבָל אִם אָמַר לוֹ שְׁלִישׁ מַה שֶּׁאַתָּה קוֹצֵר שֶׁלָּךְ, מֻתָּר בְּלֶקֶט וּבְשִׁכְחָה וּבְפֵאָה, וְאָסוּר בְּמַעְשַׂר עָנִי:
Someone who exchanges [his produce] with the poor [i.e. their gifts from the field], what is [now] his is exempt [in tithes] and what is [now] the poor person's is obligated [in tithes]. If two [poor people] received a field as sharecroppers, one may give the other <i>Ma'aser Ani</i> [a second tithe given to the poor in the the third and sixth years of the Sabbatical cycle] from his portion, and that other may give him <i>Ma'aser Ani</i> from his portion. Someone who contracts for a field to harvest, is forbidden in [taking] <i>Leket</i>, <i>Shikhechah</i>, <i>Peah</i>, and <i>Ma'aser Sheni</i>. Rabbi Yehudah said: When? At a time that he contracts for a half, a third, or a fourth [of the harvest]. But if he said to him, "a third of what you harvest is yours", [then] he is permitted in [taking] <i>Leket</i>, <i>Shikcha</i>, and <i>Peah</i> and forbidden in <i>Ma'aser Sheni</i>.