Bava Metzia4: 9
אֵלּוּ דְבָרִים שֶׁאֵין לָהֶם אוֹנָאָה: הָעֲבָדִים, וְהַשְּׁטָרוֹת, וְהַקַּרְקָעוֹת, וְהַהֶקְדֵּשׁוֹת. אֵין לָהֶן תַשְׁלוּמֵי כֵפֶל, וְלֹא תַשְׁלוּמֵי אַרְבָּעָה וַחֲמִשָּׁה. שׁוֹמֵר חִנָּם אֵינוֹ נִשְׁבָּע, וְנוֹשֵׂא שָׂכָר אֵינוֹ מְשַׁלֵּם.  רַבִּי שִׁמְעוֹן אוֹמֵר: קָדָשִׁים שֶׁהוּא חַיָּב בְּאַחֲרָיוּתָן — יֵשׁ לָהֶן אוֹנָאָה; וְשֶׁאֵינוֹ חַיָּב בְּאַחֲרָיוּתָן — אֵין לָהֶן אוֹנָאָה. רַבִּי יְהוּדָה אוֹמֵר: אַף הַמּוֹכֵר סֵפֶר תּוֹרָה, בְּהֵמָה, וּמַרְגָּלִית — אֵין לָהֶם אוֹנָאָה. אָמְרוּ לוֹ: לֹא אָמְרוּ אֶלָּא אֶת אֵלּוּ.
The following things are not governed by [the law of] fraud: Slaves, deeds, land, and consecrated articles. [The rule of] two-fold payment does not [apply] to them, nor [that of] the fourfold and fivefold payment. An unpaid shomer need not take an oath, nor need a paid shomer pay.  R’ Shimon says: Consecrated articles for which one is responsible are governed by [the law of] fraud; but those for which he is not responsible are not governed by [the law of] fraud. R’ Yehudah says: Also one who sells a Torah scroll, an animal, or a pearl is not governed by [the law of] fraud. They said to him: They stated only these.
Bava Metzia4: 10
כְּשֵׁם שֶׁאוֹנָאָה בְמִקָּח וּמִמְכָּר, כָּךְ אוֹנָאָה בִדְבָרִים. לֹא יֹאמַר לוֹ: ,,בְּכַמָּה חֵפֶץ זֶה?” וְהוּא אֵינוֹ רוֹצֶה לִקַּח. אִם הָיָה בַעַל תְּשׁוּבָה — לֹא יֹאמַר לוֹ: ,,זְכֹר מַעֲשֶׂיךָ הָרִאשׁוֹנִים!” אִם הוּא בֶן גֵּרִים — לֹא יֹאמַר לוֹ: ,,זְכֹר מַעֲשֵׂה אֲבוֹתֶיךָ!”, שֶׁנֶּאֱמַר: ,,וְגֵר לֹא־תוֹנֶה וְלֹא תִלְחָצֶנּוּ”.
Just as there is wronging in buying and selling, so is there wronging with words. One shall not say to another: “How much does this article cost?” if he does not wish to purchase [it]. If he was a penitent, he shall not say to him: “Remember your former deeds!” If he was the son of proselytes, he shall not say to him: “Remember your ancestors’ deeds!” for it is said (Exodus 22:20): You shall neither wrong a proselyte nor oppress him.