Halakha Mazmur 37:21: Shulchan Arukh, Choshen Mishpat & Kitzur Shulchan Arukh

15. If Reuven sold Shimon an unsecured field and Yehuda brings a claim against Shimon to remove the field from his possession, Reuven may not testify for Shimon even though the field was unsecured because Reuven wants the land to stay in Shimon’s possession so that Reuven’s creditor may collect a debt and Reuven will not be considered a “wicked borrower who does not pay back.” This is only applies where Yehuda’s claim is via the seller. If, however, the claim is via the buyer in that Yehuda says that Shimon owes him money, then Reuven may testify for Shimon. (Tur)

A borrower is forbidden to take the loan, and spend it unnecessarily, and possibly lose it, and thus the lender will be unable to collect it. This applies even if the lender is very wealthy. One who does this is called a wicked person, as it is said, "The wicked borrow and do not repay."¹²Psalms 37:21. The Sages have commanded: "Your friend's property should be as dear to you as your own."¹³Avos 2:17. If the lender recognizes that the borrower has this character, that he has no consideration for other people's property, it is better not to lend him, than to lend him and be compelled to press him later, thereby violating on each occasion the precept, "You shall not be to him like a creditor."