Answer

Hello.
First of all, when a person says that Reuven is a liar, that does not disqualify Reuven. At most, it prevents that person from using Reuven as a witness, along the lines of: "According to you, he is a liar." But even that is not really correct, since Reuven is not disqualified as a witness unless that has been determined by a religious court. So what this does is only prevent the use of Reuven's testimony, not disqualify Reuven himself. And the reason is that testimony which, according to your own position, is false cannot be used by you. Your own admission as a litigant says it is false, and a litigant's admission is like a hundred witnesses. But here I am not making use of the part of the testimony that is false according to my position (that he repaid), and regarding which I have effectively admitted as a litigant; rather, I am using the other part (that he borrowed). Therefore there is no problem.
And in general, testimony that disqualifies a person cannot disqualify the person intrinsically; at most it can negate the admissibility of his current testimony. A person's disqualification as a witness has to be established by a religious court, and therefore it is relevant only from the next testimony onward. So, for example, when a person comes under suspicion for a transgression, the testimony that implicates him does not disqualify him except with respect to that very matter itself (that is, it disqualifies his statement, not him personally). He is disqualified only from that point forward. I think this is the reason that when a person comes and says, "I borrowed with interest from so-and-so" (Bar Binitus, Sanhedrin 25), we split the testimony and accept his testimony about the other person, even though in that testimony he makes himself into a wicked person. The reason is that what is disqualified in the incriminating testimony itself is only the testimony, not the person; and regarding the testimony, what is inadmissible is only that he was the borrower, but not that there was a loan. So too in our case: even if for some reason I were to disqualify the testimony that he repaid, I would split it and accept the testimony that he borrowed.
True, there is a rule that if part of testimony is invalid, all of it is invalid (see the well-known comments of the Raavad and other medieval authorities cited in the Rosh, Makkot 7a, who limits the rule of splitting testimony only to a litigant himself), but that is when the testimony is invalid. Here, by contrast, I have admitted against that part, and then it is merely unusable for me.
A slightly different formulation:
The dispute between the lender and the borrower is only over the question of whether there was a loan. They both agree that there was no repayment. Therefore the testimony is heard and entered into the record only regarding that part. And here there is no obstacle to using the testimony. The one who cannot make use of the witnesses is דווקא the borrower, who wants to rely on them in order to claim that he repaid. But here there is a litigant's admission that he did not repay, and therefore even if they are believed, he will not win the case. In the past I explained that this is not really a litigant's admission (since he did not intend to admit), but rather a legal principle: what you do not claim and do not seek, you also cannot bring evidence for in religious court. The parties' claims define the framework of the case, and the proceeding will not go beyond the framework they set.
Therefore, when the witnesses come and want to testify that he borrowed and repaid, the religious court will stop them and will not allow them to testify that he repaid, because there is no litigation about that. The novel point is that even if the borrower now wants them to testify that he repaid, they are not believed. But it is not a matter of their having testified that he repaid and us refusing to accept that testimony.