Q: My friend claims that I owe him $500 for expenses on a trip that we took together. What if I claim that I covered my own expenses or reimbursed him already? What if I admit that I owe $300?

A: The first cases are ones of denial (kofer hakol). Since you are in possession of the money, the burden of the proof is on the plaintiff (hamotzi meichaveiro alav hare’aya). Therefore, you are exempt unless your friend can prove that you owe him. However, Chazal require you to take a simple oath (shevuas heses) that you do not owe him (C.M. 75:7).

The last case is one of partial admission (modeh b’miktzas); you admit $300 of the $500 claim. This is one of the three instances in which the Torah obligates an oath. Thus, you must take a severe oath that you owe only $300, and would only then be exempt from the remaining $200 (75:2).

Nowadays, we strongly discourage taking oaths; beis din would encourage a compromise in lieu of the oath. In the first case of a simple oath, it would be tilted in your favor, whereas in the latter case of a severe oath, it would be tilted in your friend’s direction.