Sefer HaChinuch ספר החינוך

The commandment to press the foreigner: To press the foreigner that worships idolatry, to pay what he is obligated to us. And we should not have pity and not have mercy upon him, to prolong the loan. And about this is it stated (Deuteronomy 15:3), “You shall press the foreigner.” And so did they say in Sifrei Devarim 113, “‘You shall press the foreigner’ — this is a positive commandment.”

It is from the roots of the commandment [that it is] so that we not teach our souls to have pity and to have mercy upon them, so that we are not pulled after their deeds and their counsel in anything. And Ramban, may his memory be blessed, wrote (in Sefer HaMitzvot, Shorashim 6) that this is not a positive commandment at all; but rather the matter of the verse is to warn us [to have] pity towards the Israelite with a positive commandment and a negative commandment: And it stated, “Press the foreigner,” but not “your brother.” And [so] it is a negative commandment that comes from the implication of a positive — about which it is established for us that it is like a positive commandment. And the negative commandment is also elucidated by it, “and you shall not press your brother.” And so is the matter exactly with the prohibition of interest, like we will write in the Order of Ki Tetseh (Sefer HaChinukh 573).

The laws of the commandment are included in the elucidation of the Scripture, according to my opinion.

And this commandment is practiced in every place and at all times by males and females. And one who transgresses it and prolongs the time for repayment from a foreigner from the angle of pity upon him alone — and not from the angle of fear of the borrower, or [fear to] lose the debt, or for any other benefit — has violated a positive commandment, according to the opinion of Rambam, may his memory be blessed. But according to the opinion of Ramban, may his memory be blessed, one who presses an Israelite for his debt has violated this positive commandment, besides that he violated a negative commandment. But prolonging the debt of a foreigner “does not raise or lower [a thing].”