Sa’if 4, Mechaber: Some are of the opinion that a borrower who says he already repaid the loan earlier on the day when the loan fell due is only believed if the lender made his claim at the end of that day. The borrower is not, however, believed if he raises this defense before the end of the day. In addition, if, in response to a claim by the lender at the end of the day, the borrower says he repaid the loan before the maturity date, he is believed on the basis of a migo. If he were lying, he could have raised a better defense. He could have claimed he repaid the loan earlier on the day that the loan fell due.
NER EYAL: It seems that all agree that where the lender sues the borrower for repayment of an oral loan on its maturity date, the borrower may, at any time during that day, raise the defense that he already repaid the loan earlier that day. This defense is not restricted to being raised only at the end of the day.
What the Mechaber is referring to here when says some are of the opinion the defense may only be raised at the end of the day is the defense mentioned in the third sentence of this sa’if, namely, when the borrower responds that he repaid the loan prior to the maturity date.
Such a defense can only be raised at the end of the day because it is based on a migo. The migo is that if the borrower were lying, he could have come up with a better defense. He could have claimed he repaid the loan earlier on the day when the loan fell due. But this migo defense in turn would only be credible if raised at the end of the day the loan fell due, when it is feasible that the lender may have forgotten the borrower repaid him earlier that day. This migo defense would not be credible if it were raised earlier that day. It is unlikely the borrower would have had the gall to maintain that he repaid the loan such a short time ago and the lender already forgot about it.