Similar concepts apply in the following situation. The plaintiff claims: "I lent you a maneh and here is one witness who will testify that this is so." The defendant responds: "That is true, but you owe me a maneh to match it." The defendant is obligated to take an oath, but cannot take that oath, and hence, is obligated to pay.
Why can he not take an oath? Because he acknowledges the content of the testimony of the witness. And a person who must take an oath because of the testimony of one witness may take the oath only when he contradicts the witness, denies his testimony and takes the oath to support his denial.
Similarly, when there is a promissory note signed by one witness and the defendant claims to have paid the debt, or a person denied a claim, a witness testified against him, and then the defendant stated that he paid the debt or returned the entrusted article, ? the defendant is obligated to take an oath, but may not take the oath. Hence, he must pay.
An incident once occurred concerning a person who seized a slab of silver from a colleague in the presence of one witness. Afterwards, he said: "I seized it, but what I seized was mine." Our Sages said: "He is obligated to take an oath, but may not take the oath. Hence, he must pay. Similar principles apply in all analogous situations.