Psalms 37:21 refers to someone who borrows money and does not repay. Jeremiah was referring to somebody who is giving Tzedakah; a donation which he did not expect, or indeed request, the recipient to return. Yirmiah was praying that the people of Anatot (see Tosfos DH Afilu) should not receive reward for the charity that they gave. This is because the recipients will be unworthy people and will presumably do bad things with the money they received, so this is considered a stumbling block for the donor. The person who received the loan referred to in Tehilim 37:21 may have used the money for very good purposes but he did not pay back as he had promised to do.

Sefer Chasidim's explanation of "unworthy paupers":-

I found, bs'd, support for what I wrote above in Sefer Chasidim (by Rabeinu Yehudah HaChasid) #61. He writes that it is much better to give to G-d fearing paupers; who lost their property; than to give to unworthy paupers (he uses the words "She-Ainam Mehuganim", which are the same words used in our Gemara by Rava to describe Yirmiah's tefilah). He writes that it is considered a crime to give money to immoral people who then use it to hire harlots and perpetuate in the world those who rebel against Hash-m. This is why one should daven to Hash-m that he will send him worthy poor people so he can perform the Mitzvah of Tzedaka properly.

The above Sefer Chasidim explains clearly what the stumbling block is; he donates money which promotes bad causes. In contrast the person that Tehilim 37:21 describes lending money to, may be a moral person apart from the fact that he does not return his debts.

Dovid Bloom

Pnei Shlomo ( written by author of Kitzur Shulchan Aruch) writes that the unworthy poor are those who are not embarassed about receiving charity:-

Pnei Shlomo on Bava Basra 9b (where the Gemara also cites the saying of Rava that it cites here) writes that we see with our own eyes that there are poor people who are not at all embarassed to receive Tzedakah. On the contrary, they often argue and take the Tzedakah forcibly. This is a sign that they are not worthy recipients.

Pnei Shlomo does not go so far as Sefer Chasidim; that I cited above; and say that the unworthy recipients actually spend the money for sinful purposes, but he appears to learn that since their behavior is arrogant, if someone gives them charity this is not considered as a Mitzvah. It seems that they are a stumbling block since the person who gives them donations thinks he is doing a Mitzvah but in reality he is not.

Sometimes not getting the money back from a loan can be worse than being tricked by charity beggers:

I posed this question to an older Talmid Chacham and he gave an answer on practical lines. Usually one does not give such a large sum to any individual request for Tzedaka whilst it is more frequent that one lends a large amount to an acquaintance who one mistakenly believes is reliable. When he does not pay back one loses a lot of money, whilst one does not usually lose so much money even if the person you gsve the Tzedaka turns out to be a trickster. That is why the person who does not return the money is called a Rasha, which seems to be worse than the term "She-Ainam Mehuganim" that our Gemara uses.

[It so happens that the Talmid Chacham, who gave me this answer, told me a while back that somebody owes him a lot of money and has not returned it. So he is speaking from personal experience!]

Dovid Bloom