In laws of loans chapter 13 law 4, Rav Shach brings an important idea that once you have one side take an oath, the other side does not have the option of saying that they do not accept it's validity. However, in laws of marriage chapter 16 law 25 at the he himself brings an important question on this approach. He brings the case where there are two sets of witnesses concerning two documents, and each set contradicts the other, and the law is you have the person take an oath. Rav Shach himself mentions there that this contradicts the idea that you have to accept the oath plus other questions he asks there. I guess one could say it is a case of "sugiot halukot", different places in shas where the sages in one place contradict the sages in another place. [I have used that idea to answer the question of the contradiction between Gemara in Shabat and the Gemara in Pesachim concerning the time of Rabainu Tam and the beginning of the night.]------------------------------------------In הלכות of מלווה ולווה chapter 13 law 4, רב שך brings an important idea that once you have one side take an oath, the other side does not have the option of saying that they do not accept it's validity. However, in הלכות of אישות chapter 16 law 25 at the he himself brings an important question on this approach. He brings the case where there are two sets of witnesses concerning two documents and each set contradicts the other, and the law is you have the person take an oath. רב שך himself mentions there that this contradicts the idea that you have to accept the oath, plus other questions he asks there. I guess one could say it is a case of סוגיות חלוקות, different places in ש''ס where the אמוראים in one place contradict the אמוראים in another place. [I have used that idea to answer the question of the contradiction between גמרא in שבת and the גמרא in פסחים concerning the time of רבינו תם and the beginning of the night.]----HOWEVER, it occurred to me that there is a better answer. The Rambam adds the reason borrower takes an oath is that he admits to part of the obligation. Clearly, in any case where the lender does not take an oath(note 1), and the borrower thus can say, “I do not know if he still has the pledge”, the borrower would not have to take an oath (since he does not admit to part of the obligation,(He admits to the part that he knows that he owes, but the other part he is not saying that he does not owe it. Rather he claims he does not know if he owes it.) and thus he would simply pay what he admits he owes, the 1/4 of the loan, and he has no further obligation. (note 1) E.g., the case in laws of borrowing chapter 6 law 1 where the lender agress to pay for the pledge and it is readily availible to buy one just like it in the market