Halakhah zu Tehillim 37:21

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)