The Mishna on our daf discusses someone who found a type of shetar –
Gitei nashim – divorce documents
Shiḥrurei avadim – a document that frees a non-Jewish slave
Dyatiki – a document that records property willed by someone on their deathbed
Matana – a document that records presents given to others
Shovrin – receipts written by a lender to a borrower
According to the Mishna, in all of these cases the document cannot be returned to either of the people mentioned, because it is possible that the individual who arranged for the document to be written changed his mind and did not do so. If that is the case, then what was written in the document never took effect, since a divorce or a present, for example, only takes place when the document is handed over by the person who has the power to effect the divorce or the present. If the document is given to them by the finder, the recipient may lie and use the document as proof to the falsehood.
Although the Mishna only gives a reason for not giving the document to the intended recipient, it is clear that the document can also not be given to the person who wrote the shetar, since, as the Meiri explains, it is likely that the document really was handed to the intended recipient properly, and that he is the one who dropped the shetar.
In truth, if the shetar was returned to its author, the situation would ordinarily be no worse for the intended recipient than if the finder holds on to it himself, since, in any case, without the proof of a legal document, the woman will have to bring witnesses to prove that she is divorced, the slave will have to bring witnesses and so on. Nevertheless, the Mishna teaches that it cannot be returned to its author, since we only return an object that was found to the person who we are convinced it belongs to, and there is no clear indication of ownership in this case.