We are forbidden to pressure those who owe us money for repayment if we know that they do not have the means available. Lending someone money in the first place was an act of kindness, helping him out in his time of trouble. Acting like a collection agency towards him turns the favor into a huge disservice. By pressuring the borrower, the lender goes from being part of the solution to being part of the problem. That kind of help is no help at all.
The Talmud in Baba Metziah (75b) tells us that if we know that someone does not have the ability to repay us, we may not even pass by his house so that he won’t think we’re harassing him. (Even if such is not our intent, he’ll feel harassed, so the bottom line is the same!)
While violation of most negative commandments was punishable with the penalty of lashes, this one was not. The lender had “plausible deniability” to say that it wasn’t his intention to harass the borrower or that he was unaware that the borrower lacked the funds to repay. Since it was unenforceable by the courts, this mitzvah is one where God takes care of things, as He knows the lender’s true motivations.
In his Sefer HaMitzvos HaKatzar, the Chofetz Chaim points out that it is not only forbidden for a lender to pressure a borrower without means, it is also forbidden for a borrower who is able to repay to refrain from fulfilling his obligation. To support this, he cites verses from Mishlei (Proverbs 3:28) and Tehillim (Psalms 37:21) on the evil of refusing to repay a loan if one is able.
This mitzvah applies to both men and women, in all times and places. It is discussed in the Talmudic tractate of Baba Metziah on page 75 and in the Shulchan Aruch in Choshen Mishpat 97. It is #234 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos and #52 of the 194 negative mitzvos that can be fulfilled today as listed in the Chofetz Chaim’s Sefer HaMitzvos HaKatzar.