One of the ways of ensuring the veracity of a legal document was to have the signatures of the witnesses examined and approved by the Jewish court. Once the beit din put its formal stamp of approval on the document, it will be considered valid with no need for any further verification.
The Gemara on today’s daf discusses a situation where space is left above the court’s stamp of approval, a situation where ordinarily the empty space is filled in with lines of ink. The Gemara asks whether we must be concerned with the possibility that the individual holding the document may cut off the text of the document together with the witnesses’ signatures, erase the lines and write a false promissory note together with the signatures of forged witnesses directly above the court’s seal of approval. In such a case the forged document could be presented for collection, given Rav’s ruling that a document – including the witnesses’ signatures – can be written on parchment that had been erased. The Gemara responds that when the parchment was erased we will require personal testimony of the witnesses and will not accept the seal of approval without carefully checking into the situation.
A question that is raised by the rishonim is why we are only concerned about an erased document in this case, and not in other cases where the witnesses’ signatures have been approved. Perhaps the individual holding the document will erase it and rewrite it, leaving the approved signatures to attest to the contents of the document.
The Rashbam essentially accepts the question and suggests that the question would be raised in all cases of legal documents. Tosafot argue that we do not suspect that people will attempt to erase letters and words; the question only comes up where the erasures will be simple lines, which are easy to undo. Tosafot Ri”d offers another suggestion. In this case, the person holding the document has nothing to lose. His original document remains in force even as he tries to steal by means of a forged document. In this case, someone who erases a real document is taking a chance that he will be discovered and will receive neither his attempted thievery, nor the money that was really owed to him, since the document has been destroyed.