Sa’if 5, Mechaber: Reuven sues two borrowers who borrowed from him jointly. The loan is not documented. One of the borrowers denies the claim. The other borrower admits the two defendants borrowed from Reuven jointly as partners. The halacha is that the borrower who admitted the claim is liable to pay the total amount claimed. However, his testimony that the other defendant borrowed jointly as partner is not admissible. Accordingly, it cannot be used to oblige the other defendant to take a Torah oath as is usually the case where one witness testifies against the defendant. The reason for this is that the defendant who admitted the claim is not an impartial witness.
Similarly, if there were three defendants and two of them admitted the claim on behalf of all three and one of them denied it, their testimony is not admissible against the third defendant, because they are not impartial witnesses.
The result would be different, however, if in response to a promissory note produced by Reuven specifying the amount lent to each of the borrowers, each borrower claimed he repaid his portion of the debt in the presence of the other borrowers. In that case, provided there is no partnership relationship between the three borrowers, the testimony of each that the other borrowers repaid their portion of the loan is acceptable because it is impartial. The court is not concerned that the three borrowers may have coordinated their testimony. (See Choshen Mishpat, Siman 37: 4, 5, and 6.)
Ner Eyal: The testimony of the defendant who admits the undocumented loan is inadmissible against the defendant who denies it. This is because the former has a vested interest in a judgment against the latter, for if the court accepts the testimony of the former, he will be able to recoup half of the loan from the latter.
The plaintiff will only be allowed to proceed against the defendant who admitted the loan. This is the case even if the assets of the defendant who admitted the loan are less valuable than the defendant who denied it, or even if the former has no assets to collect from at all. The lender will not be able to argue that since the borrower who admitted the loan will not be paying anything anyway because he has no assets to collect from, his testimony against the other borrower is impartial. This argument will fail because the borrower who admitted the loan may come into money in the future.
The forgoing also applies to a plaintiff who sued two borrowers based upon a promissory note for the aggregate amount of the loan advanced to both of them and signed by them jointly. For example, the note is for $100,000 and it is signed without specifying that each borrowed $50,000. One of the defendants admits the amount written in the note was not repaid by him or by the other defendant. The other defendant challenges the claim and says he has repaid it or the promissory note has not been certified by the court and has been forged. As with the case of the undocumented loan, the testimony of the defendant who admits the amount written in the note was not repaid, either by him or by the other defendant, is inadmissible against the defendant who denies it. This is because the former has a vested interest in a judgment against the latter, for if the court accepts the testimony of the former, he will be able to recoup half of the loan from the latter.
The result would be different if the promissory note specifies the amount for which each borrower is liable. For example, the note states Shimon is liable for $50,000 only and Levi is liable for $50,000 only. In such a case, provided that Shimon and Levi are not partners, neither of them will have a claim of recoupment against the other, because each is only liable for the amount entered against his own name in the note. Neither of them is liable for the amount entered against the other borrower’s name. Accordingly, the testimony of Shimon, who admitted the amount written in the note was not repaid by him or by Levi, is impartial and will be accepted. Levi will now have to take a Torah oath in support of his defense, just like any other defendant confronted by one witnesses testifying against him.