Rambam concludes by stating that any and all who mistreat a ger, whether it be or in monetary dealings, verbally violates three negative commandments. First, as it states (Leviticus 25:17), “Velo tonu ish et amito [veyareita meElokecha ki Ani Hashem Elokeichem] – You shall not aggrieve one another [and you shall fear your G-d because I am Hashem your G-d.]” Rashi ad loc. explains that this command refers to ona’at devarim, verbal aggrieving.
Second, one is also in violation of the following prohibition: “[Vechi timkeru mimkar le’amitecha o kanah miyad amitecha] al tonu ish et achiv – [And when you sell goods to your brother or when you buy from the hand of your brother] you may not defraud (or aggrieve) one another” (Leviticus 25:14). Rashi explains that this command refers to monetary fraud, ona’at mammon.
Third, the wrongdoer transgresses the words of Exodus (22:20), “Ve’ger lo toneh [ve’lo tilchatzenu, ki gerim heyitem be’eretz mitzrayim] – You shall not do wrong to a stranger, nor shall you oppress him [for you were strangers in the land of Egypt.]” Rashi ad loc. derives from this statement the prohibitions of both ona’at devarim and ona’at mammon in dealing with a ger.
Thus it is clear that a convert and one who was born a Jew are to be treated equally, except that we find somewhat of a modification to that rule in another statement of Rambam. The second statement is found in Hilchot Isurei Bi’ah (12:17), where Rambam cites the Talmud (Kiddushin 69a and Yevamot 76b-77b) stating, “All gentiles, who convert and accept upon themselves the mitzvot of the Torah, or slaves when they are emancipated, are [considered] Jews in all matters, as the verse states (Numbers 15:15), ‘Hakahal chukah achat lachem [vela’ger hagar, chukat olam le’doroteichem, kachem kager yihyeh lifnei Hashem] – There shall be one law for your congregation [both for you and for the proselyte who dwells (with you), an eternal law throughout your generations, like you shall be the proselyte before G-d].’ They are permitted to enter the congregation of G-d immediately.”
This refers to a convert and an emancipated slave being able to marry a Jewish woman, or a giyoret and an emancipated bondswoman being able to marry a Jewish man, with the exception of converts from four specific nations. Converts from the nations of Ammon, Moab, Egypt and Edom are considered Jews in all matters, but they are not allowed to enter the congregation of Israel in marriage [as Rambam explains in further discussion].
Thus we see that the convert may be subject to restrictions to which the born Jew is not subject. However, even then we must treat the ger as we treat other Jews in social and financial matters, and Rambam did not intend to contradict any halacha found in the Gemara.