Sai’f 3, Mechaber: The lender sues the debtor on the very day the loan falls due. The borrower responds that he already repaid the loan earlier that day. If the loan in question was an oral loan and the borrower swears a Shevuat Heiset oath of denial that he does not owe the lender anything, he is believed. This is because it is not unusual for a debtor to repay a loan on the very day it falls due. If the loan is evidenced by a promissory note, it is treated at law as if it were a loan with no stipulated maturity date.
NER EYAL: It is not unusual for a debtor to repay his debt on the very day the loan falls due. Accordingly, if the lender sues for repayment of an oral loan on the day it falls due, the borrower will be believed when he says he already repaid the loan earlier that day. He will, however, have to take the Shevuat Heiset oath of denial, like any other defendant who totally denies a claim.
However, the defendant will not be believed if he says he repaid the loan at the very beginning of the day it fell due, before business hours, because practically speaking he would not have had the opportunity to do so. In that situation, the only plausible explanation would be that he repaid the loan on the day prior to the maturity date. As we have seen, a debtor is not believed if he says he repaid the loan prior to the maturity date. The debtor is believed, however, when he says he repaid the loan earlier, during business hours, on the day it fell due.
If the lender sues for repayment of a loan evidenced by a certified promissory note on the day it falls due, the borrower cannot say he repaid it earlier during business hours that day. This is because there is no such defense to a certified promissory note. It is assumed that if the borrower had indeed repaid the note earlier that day, he would either have taken the note back from the lender or asked for a receipt of payment. If he did neither, he must pay.
However, if the borrower is sure he repaid the note earlier that day, he may insist that before he pays again, the lender take an oath that he has not already been repaid. This oath, known as shevuat hadayanim, is imposed by the rabbis on the plaintiff as an extension of the modeh memiktzat oath of partial admission, imposed by the Torah on a defendant who admits part of a claim and denies part of the claim. This shevuat hadayanim oath obliges the plaintiff, before collecting payment of the note from the defendant, to swear in the name of God, while holding a Sefer Torah, Tefillin or some other holy article containing passages from the Bible, that he did not already receive money from the borrower in repayment of the loan.