The 253rd prohibition is that we are forbidden from causing financial damage to a convert.
The source of this prohibition is G‑d's statement,1 "Do not oppress him."
In the words of the Mechilta: "The verse 'Do not oppress him' means to do so financially."
It has already been explained in tractate Bava Metzia2 that one who [verbally] causes distress to a convert transgresses the prohibitions "V'lo sonu one another"3 and "Do not wrong a convert."4 One who oppresses him [financially] transgresses the prohibition, "Do not oppress him," in addition to the general prohibition that includes all Jews, ona'as mamon.5