Commentary for Bava Metzia 4:10
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 (שמות כב) וְגֵר לֹא תוֹנֶה וְלֹא תִלְחָצֶנּוּ:
Just as there is ona'ah in buying and selling, so there is ona'ah in words, [it being written (Leviticus 25:17): "And you shall not wrong, one man his fellow, and you shall fear your G d." This speaks of verbal wronging, the good or evil intent of one's words not being manifest, but known only to the speaker.] One should not ask another the price of an article if he does not intend to buy it. If he were a penitent, he should not say to him: "Remember your past deeds." If he were the descendant of proselytes, he should not say to him: "Remember the deeds of your ancestors," as it is written (Exodus 22:20): "And a stranger you shall not wrong, and you shall not oppress him."
Bartenura on Mishnah Bava Metzia
English Explanation of Mishnah Bava Metzia
For it is said (Exodus 22:21), “And a stranger you shall not wrong or oppress.”
Just as a person can wrong another person by cheating him on a sale, so too can a person wrong another person through words. One may not ask someone how much something costs he doesn’t intend to buy it. This would raise false hopes for the seller. One may not remind a person who had repented of his former deeds. Nor may one remind a convert that his ancestors were idol worshippers. These laws are learned from a midrash on the verse in Exodus 22:21. The verse uses the word “ger” which in Biblical Hebrew means stranger but in Rabbinic Hebrew means convert. The word for “wrong” in this verse is the same word used in Leviticus 25:14 (see above) from where we learned the laws of fraud. The Rabbis extended the “wrong” mentioned in the verse to include even wrong done through words alone.