Shevuos, Chapter 8, Mishnah 3. The Mishnah continues to deal with an unpaid guardian, clarifying in which circumstances he is liable to bring a korban for a false oath and in which circumstances he is exempt.
One who denies that he ever guarded at all:
The owner sues his fellow, claiming that he was an unpaid guardian of his ox, and asks him: "Heichan shori?" - "Where is my ox?" The defendant answers: "Eini yodea al mah atah medaber" - "I have no idea what you are talking about," meaning: I was never an unpaid guardian for you and I never guarded your ox at all. In reality, however, he was indeed a guardian, and the ox died, was injured, was captured, was stolen or was lost. The claimant said to him, "I impose an oath upon you," and he swore - he is exempt from the korban for a false oath.
The reason for the exemption: even according to the facts as they truly were, he was exempt from paying and bore no liability at all. As we saw in the previous Mishnah, instead of stating what actually happened - a claim for which he is exempt - he made a different claim for which he is likewise exempt, namely that he was never a guardian at all. Either way, he is exempt. It emerges that there is no obligation of a korban for an oath except where a person denies a liability that rests upon him; and since there is no liability here, even though he swore falsely, this is not an oath of a deposit, though it may fall under the category of an oath of expression.
He claimed it was lost, and it turned out that he ate it:
The owner sued the guardian: "Heichan shori?" - "Where is my ox?" and the guardian answered "Avad" - "It was lost," a claim for which he is exempt. The claimant said to him, "I impose an oath upon you," and the guardian accepted the oath upon himself. Witnesses then came and testified that the guardian himself had eaten the animal - he slaughtered it and ate it. The law is: he pays the principal alone, and he does not pay a penalty and does not bring a korban. The reason: he did not admit on his own, and there is no obligation of a korban for an oath except where a person admits on his own.
But if he admitted on his own after having sworn falsely that he ate it, he pays the principal, adds a fifth, and brings an asham offering for having sworn falsely.
He claimed it was stolen, and it turned out that he stole it himself:
The owner sued: "Heichan shori?" - "Where is my ox?" and the guardian answered "Nignav" - "It was stolen," a claim for which he is exempt. The owner said to him, "I impose an oath upon you," and he swore. Witnesses then came and testified that he himself had taken it for himself - he pays the double payment, like any thief.
What is the difference between this case and the previous one?
It depends on the claim he made. One who claims that the item was lost is not obligated by the Torah to pay double for his claim, even though he swore falsely about it. But one who claims that it was stolen when in truth he himself took it is liable for the double payment. In any event he does not bring a korban for the oath, again for the reason that he did not admit on his own.
And if, after claiming "It was stolen" and swearing falsely about it, he admitted on his own, and no witnesses came, he does not pay double, since one who admits to a penalty is exempt. But he does pay the fifth, and even though it is a penalty, it is a penalty that the Torah established specifically for one who admits on his own, and of course he brings a korban for the oath.
In summary: In this Mishnah we learned that one is liable to bring the offering for a false oath of a deposit only when he denies a monetary obligation that rests upon him, and therefore one who denies the very fact that he was entrusted with the item is exempt. We also learned that liability for the offering and the additional fifth depends on the one who swore admitting on his own, whereas when witnesses come and testify he pays only the principal; and that the double payment depends on the claim he made - one who claims "it was stolen" and it turns out that he took it for himself pays double, while one who claims "it was lost" does not pay double.