(Rav Chana): We may learn [validate signatures] based on an accurate Sefer [that the witness wrote], e.g. these who say (write at the beginning of the Sefer) 'these are the Seforim of Asi.'

Question: [It is unclear whether or not we may validate signatures based on] letters (correspondence).

(R. Yosi bei R. Bun): (R. Lazar allows Kiyum even from a document that was not validated in Beis Din.) This is only from three documents that three different people hold.

(R. Chunah citing Rav): Witnesses are believed to say that it is an Amanah document (the borrower trusted the prospective lender and gave to him the document before the loan was given) or a Potisim document (it is merely to make the bearer appear rich).

(Rav): It is forbidden to sign on an Amanah document or a Potisim document.

Question: Does Rav argue with what R. Chunah said in the name of Rav, that witnesses are believed to say that it is an Amanah document or a Potisim document? Does Rav contradict himself?! (We explained this like MEICHAL HA'MAYIM.)

Answer (R. Chagai): Rav did not say that it is forbidden to sign [on such documents], rather, it is forbidden to keep them;

This is like [we expound] "Im Aven b'Yadcha Harchikehu" - this is an Amanah document or a Potisim document. "V'Al Tashken b'Ohalecha Avlah" is a paid document.