On a de'oraita (Torah law) level, if somebody has a legal document saying that they are owed money, they are allowed to collect it without taking an oath. But in certain situations, even a person with a document still has to swear. In Masechet Ketubot, they gave five examples of a woman collecting her ketubah (marriage contract) in certain scenarios where she has to swear that the money is still owed to her. We'll first begin by quoting those cases and then add one more.
The Mishnah in Masechet Ketubot says, as the Hachamim (the Sages) said, that if a woman reduces her ketubah—that is, she presents her ketubah but then says that part of it was already paid off—she would not be paid the rest of the ketubah unless she swears that she has not been paid in full. Another case is if one witness testifies against her and says that the ketubah was already paid off. Usually, one witness does not amount to anything. But in this case, she would not be paid unless she swears that she did not already receive the money.
Number three: if she comes to collect a ketubah from property that had been sold and had a lien against it—property that people bought after the ketubah debt was already in effect—then she comes to collect from that property. Or she comes to collect from her husband's estate, from property inherited by his children. She wants to collect from there. We say that in those cases as well, she can't be paid. She can't collect a ketubah from the buyers of her husband's property or from her husband's orphans unless she swears that she did not receive it—that the ketubah is still owed to her.
And then lastly, if a woman wants to be paid her ketubah after her husband divorces her, but he is not around—he traveled, he went overseas—and she wants to be paid her ketubah from his property while he is not around, we say she would not be paid unless she swears that she is still owed the money.
Likewise, the Hachamim made a decree that orphans cannot collect a debt owed to their father unless they swear. Even though orphans may have a loan document saying that their father was owed money, they cannot collect it from the borrower's children without taking an oath. Just like a woman cannot collect a ketubah from her husband's inheritors without taking an oath.
So we mentioned that children have to swear in order to collect a debt that was owed to their father. But we can't expect them to swear that they know with certainty that their father was not repaid, because how would they really know with certainty? It's impossible. So the Mishnah tells us: What oath do they really make? “We swear that our father did not instruct us or tell us on his deathbed, and our father didn't tell us at any time before then. We didn't find any receipts among our father's documents. We don't have any documents stating that this loan document has been paid.”
So they basically have to swear that their father never told them anything about this loan being paid, and that they don't find any record of it. The implication is that if they didn't have the ability to speak to their father, then they would not make any oath. Here we're saying they had the option of talking to their father, but they swear that he never said anything to them. So again, if they didn't even have the option—for example, let's say they were born after their father died—they could collect without an oath. But Rabbi Yochanan ben Berokah is going to disagree. If the lender's son was born after his father's death, he still has to swear that he finds no records of a receipt, and only then could he collect the debt, even though part of the oath can't be that he heard from his father that the loan was paid. He still has to swear that he didn't find any documentation that the loan was paid.
The next opinion, Rabbi Shimon ben Gamliel, is going to tell us whether an orphan needs to swear when there's outside evidence that the father said before he died that he wasn't paid. Rabbi Shimon ben Gamliel said that if there are witnesses who tell us that the father said right before he died that the document wasn't paid, then the son can collect without an oath. We don't make him swear at that point.