We are now learning Mishnah 7 in the tenth chapter of Tractate Bava Kamma. The Mishnah addresses the question: what is the halachah when a person accused of theft is unsure whether he is obligated to return the item? There are two scenarios before us:
He admits that he stole the item, but he does not remember if he returned it.
He is completely unsure whether he stole it at all, since he does not remember the event clearly to begin with.
The Mishnah continues and establishes that all the rules stated regarding theft apply equally to a loan - when the borrower is unsure whether he repaid it, or whether he took it at all - and similarly to a deposit, when the guardian is unsure whether he returned the deposit, or whether he received it to begin with.
The base case, which the Mishnah does not discuss:
Reuven says to Shimon: "You owe me a hundred dollars," and Shimon replies: "I do not owe you." This is not a case for our Mishnah, since there is no doubt here at all and both parties are claiming with certainty. In such a case, the Torah rule is "the burden of proof is on the one who seeks to extract from his fellow" - the burden of proof rests on the one seeking to take money from his friend's pocket. And since Reuven has no proof, he has no power to obligate Shimon to pay.
Nevertheless, there is a rabbinic law regarding this matter, since a person does not accuse his fellow of holding his money unless there is some substance to the claim - there is no smoke without fire. Therefore, the Sages instituted the "shevuas hesset", a rabbinic oath that they imposed on the defendant: Shimon indeed does not pay, but he can be compelled to swear that he does not owe the money, and only after that will he be exempt.
A claim of certainty against a claim of doubt:
Our Mishnah deals with a case where Shimon does not say "these things never happened," but rather "I am not sure." Reuven claims that his money is with Shimon and asks him to return it - this is a claim of certainty; whereas Shimon responds with a doubtful claim, "I do not know." There are two versions of this answer, and each has its own halachah.
"Ha'omer lachaveiro: gazalticha, hilvisani, hifkadta etzli, ve'eini yodea im hechezarti lecha im lo hechezarti lecha - chayav leshalem" - If one says to his fellow: I stole from you, you lent me money, you deposited an item with me, and I do not know whether I returned it to you or if I did not return it to you, he is obligated to pay:
"Gazalticha" - I indeed stole from you.
"Hilvisani" - you indeed lent me money.
"Hifkadta etzli" - you indeed placed an item in my hands to guard.
Shimon therefore admits to the obligation itself, and is only in doubt regarding its repayment: "Eini yodea im hechezarti lecha im lo hechezarti lecha" - he thinks he returned it, but he is not sure about it. The halachah is that he is obligated to pay and return the claimed money or item to Reuven, since he admits to the existence of the obligation and does not deny it, but only claims he may have already paid it. And since Reuven claims with certainty that he was not repaid, he has the power to extract it from him, and the beis din even extracts it from him in practice.
"Aval im amar lo: eini yodea im gazalticha, im hilvisani, im hifkadta etzli - patur milshalem" - But if he said to him: I do not know if I stole from you, if you lent me money, or if you deposited an item with me, he is exempt from paying:
Here, Shimon denies the very claim that he ever owed anything: he cannot confirm that he stole, that he borrowed the hundred dollars being claimed, or that money was deposited with him for safekeeping. Therefore, he is not obligated to pay, since the rule is that when a definite claim on the part of the plaintiff and an uncertain claim on the part of the defendant are pitted against each other - a definite claim is not superior, and a claim of certainty does not have the power to extract money.
However, it is clear and obvious that Shimon will be required to take a rabbinic oath of denial (shevuas hesess) that he does not owe the money, and only then will he be exempt. After all, if even someone who claims with certainty that he never owed anything is obligated to swear, all the more so that saying "I am not sure" is not a better defense claim that exempts one from an oath. Otherwise, every defendant could exempt himself from all his obligations simply by saying "I am not sure".
The Obligation to Fulfill One's Duty to Heaven:
It must be clarified that even when the court does not extract the money and the defendant is exempt, there may still be an obligation upon him to fulfill his duty to Heaven: the Almighty knows whether he owes the money, and the mere fact that he does not remember does not cancel his debt. In practice, this means:
He admits the obligation but is uncertain about the payment: He is strictly obligated to pay because of his admission, and the court extracts it from him.
He is uncertain about the obligation itself: He is exempt in court, but if he truly owes it - he must fulfill his duty to Heaven and return it, since his account is managed Above. If he wishes to settle the account and not deal with the consequences in the World to Come, he must give the money.
When There Was No Claim at All:
A case that the Mishnah does not deal with is when Reuven never claimed anything, and Shimon wakes up one morning and wonders if he owes Reuven money. Here too, we must distinguish:
If it is clear to him that he indeed owed money in the past, and he is unsure if he returned it - he certainly cannot be forced to pay, but if he does owe it, he must fulfill his duty to Heaven and return the money.
If this is nothing more than a troubling thought - "Perhaps I borrowed money from Reuven" - there is not even an obligation to fulfill his duty to Heaven here. He should let go of the obsessive thought, and he does not need to pay, since there is no claim against him at all and he is not even sure he was ever obligated in the first place.
Tosafos further emphasize: if Reuven's claim itself was stated in terms of doubt - "Perhaps you owe me money" - it has no substance, and it is as if there were no claim here at all. Therefore, even to fulfill his duty to Heaven, and even beyond the letter of the law, Shimon is not obligated to respond and pay him just to be safe. The claim must open with a definite claim, in which Reuven demands his payment with certainty, in order for there to be room to obligate Shimon in court.
In Summary: Our Mishnah deals with a defendant who responds with an uncertain claim. One who admits to the obligation itself and is in doubt only about its payment - is obligated to pay, and the court extracts it from him. One who is in doubt about the obligation itself - is exempt, since a definite claim is not superior, but he takes a rabbinic oath of denial (shevuas hesess), and if he truly owes it he must fulfill his duty to Heaven. And all of this is specifically when the claim was opened with a definite claim, since a claim argued in terms of doubt has no substance at all.