וְכִֽי־יָג֧וּר אִתְּךָ֛ גֵּ֖ר בְּאַרְצְכֶ֑ם לֹ֥א תוֹנ֖וּ אֹתֽוֹ׃
И если чужеземец пребывает с тобою на земле твоей, не делай ему зла;
Rashi
לא תונו [AND IF A STRANGER SOJOURN WITH THEE IN THY LAND], YE SHALL NOT VEX HIM — This implies vexing him with words (cf. Rashi on Exodus 22:20) — do not say to him, “Yesterday you were an idolator and now you come to study the Torah which was given from the mouth of the Almighty!” (Sifra, Kedoshim, Chapter 8 2; Bava Metzia 58b, 59b.)
Ibn Ezra
If a convert dwells with you is mentioned after the elderly. Just as Scripture has admonished you to honor an elderly Israelite though he has no power, so does it admonish you regarding a convert, who also has little power compared to you (perhaps he is considered powerless simply because he resides in your land, under your dominion).
Sforno
After the Torah warned that the Torah scholars, etc., must be shown due respect, it also warns us not to treat with disdain people who are on the lower rung of the economic ladder. וכי יגור אתך גר לא תונו אותו, even if this unfair treatment consists only of words, not deeds. The Torah continues,