A ketuba is effectively a contract – an agreement between husband and wife. What happens if this document is lost or misplaced? Our Gemara takes for granted that the obligation remains. In the event that death or divorce causes the ketuba to be enforced, then the only question is how much money was guaranteed at the time of the wedding. If the wife brings evidence that she was a betula – a virgin – she will receive the 200 dinar that is appropriate in such a case. Otherwise she will have to settle for 100 dinar, which a widow receives.
Rabbi Abbahu concludes from this that a receipt is written when the ketuba is paid. Otherwise we need to be concerned lest the woman receive payment in one court based on the testimony of witnesses and in another beit din when she presents the ketuba and demands to be paid.
The Sages disagree about whether a person who acknowledges a debt will have to pay it if the lender cannot return the promissory note to him. One approach is that he should pay and accept a receipt for the payment that was made. Others argue that it is unreasonable to force him to have to guard his receipt forever, lest the lender appear in court at a later date and demand payment based on the note. According to the first approach, this is not a great concern, because the passage in (22:7) teaches: “Eved loveh le-ish malveh – a borrower becomes a servant to the lender,” therefore we can make demands on the borrower in order to encourage lending. The rishonim point out that this disagreement about the appropriateness of writing a receipt applies not only in situations of loans, but in all similar documents where one party obligates itself to another, including a ketuba.