Rav Nachman's opinion (in the second version) is that a third party is not believed when they produce a Shtar (legal document) to testify about the status of that Shtar, once the Shtar is already introduced to the Beis Din. Rava therefore asked him a question from the Beraisa that states that a third party can bring a receipt to Beis Din that a borrower paid back a lender. This is clearly even if the Shtar is already established in Beis Din as TOSFOS (DH "Simpon") points out, because the Beraisa discusses a similar case where the receipt was written on the bottom of the original Shtar (which clearly makes it a receipt that was established in Beis Din). Therefore, Rava proves from this Beraisa that the fact that the third party is believed to say that the receipt that was already established in Beis Din is an actual receipt (not a preparatory copy) proves that the third party *is believed* to establish the status of a document in Beis Din.

All the best,

Yaakov Montrose