Shevuos, Chapter 7, Mishnah 4. The Mishnah deals with a case in which the claimant swears and collects - he takes an oath and then collects the money - because the party opposing him is suspect regarding oaths. Normally it is the defendant who would swear and be exempt, but here we are dealing with someone who is suspected of lying under oath, and for that reason he is not permitted to swear at all. Therefore the claimant is the one who swears and collects.
Who is considered suspect regarding oaths:
The Mishnah spells out which transgressions place a person in the category of one who is suspect regarding oaths: "Keitzad? Echad shevuas ha'edus, v'echad shevuas hapikadon, va'afilu shevuas shav" - How so? One who swore a false oath of testimony, one who swore a false oath regarding a deposit, and even one who swore a vain oath.
"Shevuas ha'edus" - he swore that he had no testimony to offer, and afterwards admitted that he did know testimony.
"Shevuas hapikadon" - he swore falsely that he was not holding an item belonging to his fellow, and then admitted that he had lied.
"Va'afilu shevuas shav" - even an oath that causes no harm to anyone else, where he simply swore an unnecessary and improper oath. Since it appears that he does not treat the honor of the Holy One, Blessed is He, with proper respect when he takes an oath, he cannot be relied upon to swear honestly.
Additional disqualified people who are suspect regarding oaths:
The Mishnah continues: "Hayah echad mehen mesachek b'kubya, u'malveh b'ribis, u'mafrichei yonim, v'socharei shevi'is" - if one of them was a dice player, or one who lends with interest, or one who races pigeons, or one who trades in produce of the Sabbatical year. That is, one of the litigants, and this refers to the defendant against whom the claim was directed:
"Mesachek b'kubya" - one who gambles at dice.
"Malveh b'ribis" - one who lends out his money with interest, even interest that is forbidden only by rabbinic law.
"Mafrichei yonim" - two explanations are given: one who races pigeons as a form of gambling, or one who lures other people's pigeons to himself. This disqualification is rabbinic, since people are not definite owners of the birds found in their possession.
"Socharei shevi'is" - one who conducts business with produce that grew in the Sabbatical year.
All of these are disqualified from serving as witnesses, and beyond their disqualification for testimony they are also suspected of swearing falsely. Therefore, under these circumstances, "sheknegdo nishba v'notel" - the opposing party, who presented his claim, swears that his claim is true, and on that basis he collects.
When both parties are suspect:
The Mishnah continues: "Hayu sheneihem chashudin - chazrah shevuah limkomah, divrei Rabbi Yosi" - if both of them were suspect, the oath returns to its place; these are the words of Rabbi Yosi. That is, both litigants are suspected of lying under oath. What does the phrase "the oath returns to its place" mean? The Gemara offers two explanations:
The oath returns to the one who was originally obligated in it, namely the defendant against whom the claim was directed. By law he is the one who would have sworn and been exempt from payment, except that he is suspect and cannot swear. And once the opposing party also cannot swear, the obligation to swear returns to him, and since he cannot swear, he must pay. According to this explanation, "the oath returns to its place" means that the defendant will have to pay.
The "place" of the oath is Sinai: the matter returns to the oath that all of Israel swore at Mount Sinai not to steal, "lo tigzol." It is the Holy One, Blessed is He, who will exact payment from one who denies holding his fellow's money, while the court does not become involved at all - neither to administer an oath nor to obligate payment. According to this explanation, "the oath returns to its place" means that the court does nothing.
This is the position of Rabbi Yosi. But "Rabbi Meir omer: yachloku" - Rabbi Meir says: they divide it. In a case where both parties are suspect regarding oaths, they split the money between them.