Responsa for Bava Kamma 177:13

Q. A demanded from B sixty pennies he owed him. B admitted the indebtedness, but claimed that through A's wilful neglect, he, B, was not able to collect a debt of four pounds and sixty pennies. B, therefore, demanded that A pay him four pounds. The judges ordered A to swear that he did not willfully cause any damage to B, and ordered B to swear that A did willfully cause him a loss of at least sixty pennies. After the judges rendered this decision, B put forth his claim that, in partnership with A, he, B, loaned eight and one half pounds to Gentiles, and that, without his permission or consent, A relinquished that debt to the Gentiles. A, however, claimed that B gave him permission to do with B's share whatever he would do with his own. Moreover, A claims that B informed against him; a Jewish witness, the servants of the Bishop, and the burghers, support A's claim in this respect. The judges, however, demand that their former decision be carried out and that A and B present their new claims in another suit.
A. The decision of the judges that A must take an oath to the effect that he did not cause B to lose his investment by willful neglect, is correct. But if A released B's Gentile debtors (even though B gave A a blanket permission to do with his, B's, share whatever he, A, would do with his own), A must pay B as much as B's share was worth before A released the Gentiles. B must take an oath in the presence of the Gentile witnesses that he did not inform against A, in order to counteract the testimony of the single Jewish witness.
The question is signed by R. Yedidyah b. R. Israel.
SOURCES: Cr. 52; Pr. 699; L. 373–4; Mord. B. K. 96; Rashba I, 1096; Mordecai Hagadol, p. 258a. Cf. Moses Minz, Responsa 44; Isserlein, Pesakim 209.