Q: I lent a sum of money to a coworker in the presence of a single witness and we later got into a dispute over the loan. What is the status of a loan made in the presence of a single witness?
A: Generally, the testimony of two witnesses is necessary to extract payment legally. A single witness only has the ability to require the defendant to take a severe Torah oath. (Kesubos 87b)
Therefore, if the witness supports the claim of the lender, the borrower is required to take a severe oath to contradict the witness. [We already mentioned that batei din nowadays generally avoid imposing oaths and encourage a compromise.] If the borrower is unable to take this oath, such as if he does not remember clearly, he is obligated to pay the amount stated by the witness based on the principle of mitoch she'aino yachol lishava meshalem. (C.M. 75:12)