In laws of loans chapter 13 law 4 and laws of borrowing chapter 6 law 1

In a case where a lender says the pledge was worth 1/2 of the loan it was lost through negligence and the borrower claims it was worth 3/4's of the loan, the law is the lender takes an oath that he does not have the pledge anymore, and the borrower takes an oath about the value of the pledge, and the borrower pays the 1/4 of the loan that he admits to. The Ramban asks why would this be a case of admission in part? Why not let the borrower claim perhaps the lender still has the pledge and thus is not obligated to pay anything until he gets it back? Rav Shach answers that once you have the lender take an oath, you do not have the possibility to say you do not believe him. I asked on this from a case in laws of borrowing chapter 6 law one where the Rambam says once the lender agrees to pay for the pledge and a similar type is available in the market, he does not have to take an oath. And thus, the question of the Ramban returns. However, I can answer this question. In our case, there is a disagreement about the value of the pledge. It is only in a case where the lender would agree to forgo on the payment of the loan according to what the borrower claims was the value of the pledge, that the borrower would have to believe the lender.----------------------In a case where a lender says the pledge was worth 1/2 of the loan it was lost through negligence and the borrower claims it was worth 3/4's of the loan, the law is the lender takes an oath that he does not have the pledge anymore, and the שואל takes an oath about the value of the pledge, and the שואל pays the 1/4 of the loan that he admits to. The רמב''ן asks, "Why would this be a case of admission in part? Why not let the שואל claim, "Perhaps the lender still has the pledge, and thus is not obligated to pay anything until he gets it back?" רב שךanswers that once you have the lender take an oath, you do not have the possibility to say you do not believe him. I asked on this from a case in הלכות שאלה ופיקדון פרק ו' הלכה א' where the רמב''ם says once the lender agrees to pay for the pledge, and a similar type is available in the market, he does not have to take an oath. And thus, the question of the רמב''ן returns. However, I can answer this question. In our case, there is a disagreement about the value of the pledge. It is only in a case where the lender would agree to forgo on the payment of the loan according to what the שואל claims was the value of the pledge, that the שואל would have to believe the lender.