Ketubot 82A

דלמא רבי נתן היא. דתניא, רבי נתן אומר: מנין לנושה בחבירו מנה, וחבירו בחבירו, מנין שמוציאין מזה ונותנין לזה ⁠— תלמוד לומר: ״ונתן לאשר אשם לו״.

However, this still does not prove conclusively that the baraita is corrupt, as perhaps it is the opinion of R' Natan. As it is taught in a baraita that R' Natan says: From where is it derived that in the case of one who claims 100 dinars of another, and the other claims money of another, that one appropriates the money from this one, the last borrower, and gives it to this one, the first lender, without each party claiming the money from the one with whom he did business? The verse states: “And he should give it to him in respect of whom he has been guilty” (Numbers 5:7). The words “whom he has been guilty” are expounded to mean that the borrower pays the one who is owed by his creditor, since the borrower is a party to this case despite the fact that he never incurred direct liability to him. It is possible to explain the baraita cited by Rav Yosef based on this reasoning as well.

אלא: לא אשכחן תנא דמחמיר תרי חומרי בכתובה, אלא אי כרבי מאיר אי כרבי נתן.

Rather, a different justification exists for rejecting the baraita: We have not found a tanna who is stringent with these two stringencies with regard to a marriage contract. Rather, one rules either in accordance with the opinion of R' Meir that movable property is mortgaged for a marriage contract, or in accordance with the opinion of R' Natan. No one accepts both of these stringencies, and yet this baraita can be explained only by a combination of the two opinions. It must therefore be rejected as non-authoritative.