Sa’if 8, Mechaber: The plaintiff claims the defendant grabbed money from him. The defendant denies it. The defendant will prevail, provided he swears the Shevuat Heiset oath of denial. If the defendant responds to the claim by admitting he grabbed part of the money claimed or if there is one witness who testifies that he saw the defendant grab the money from the plaintiff, the defendant must swear the Modeh Bemiktzat oath of partial admission in order to prevail.
Ner Eyal: Even though the plaintiff accuses the defendant of grabbing his money, he is still eligible to take the Shevuat Heiset oath of denial if he denies it completely. Similarly, even though the defendant admits he grabbed part of the money or even if one witness testifies that he saw him grab the money, the defendant is still eligible to swear the Modeh Bemiktzat oath of partial admission. The court does not say he is a thief who is barred from taking an oath. Only a person who grabbed the money in the presence of two witnesses who testify to this fact is a confirmed thief who loses the credibility to take an oath. A person who is merely accused of stealing but denies it remains eligible to take an oath.
So too a person who admits he stole part of the money claimed, or who is confronted by only one witness who testifies to the theft, remains eligible to take an oath. This is because the court can ascribe to him an honest motive for grabbing the money. It could be that the defendant suspects, but is not certain, that the plaintiff owes him money. In order not to lose the passing opportunity of collecting the unconfirmed debt, he grabs the money first and then checks his books and records later. Even as he grabs the money, he has every intention of returning it if, after consulting his books and records, he sees the loan was indeed repaid. Although the defendant might be prepared to take the risk of grabbing money he is not certain is owed to him, he would not risk swearing an oath out of doubt. Money grabbed can always be returned, but swearing falsely has irrevocable consequences.
Accordingly, in order to discourage this type of self-help behavior, the rabbis imposed an oath on him so that he knows that grabbing will not help him unless he is certain he is owed the money and is prepared to back it up with an oath.