Bava Batra 171: Postdated and Predated Documents

The rabbis question whether a scribe can rewrite a document for sale or only for gifts.  The Gemara tells of a situation where one pays half of what he owes.  What happens next is up for debate: is a new document written from the original date?  Or does the lender keep the old document while a receipt is written for the borrower?  Would a receipt encourage the borrower to pay more quickly, for he would be fearful of losing that receipt and owing the entire amount?  Is the date written as the current date?  It could be postdated (which we would know if it were dated as Yom Kippur or Shabbat).  Or would documents postdated for a Shabbat be invalid unless they explicitly state that they are postdated?

Would a borrower be forced to accept a receipt?  The rabbis go into some detail debating this point.  In the end, it is suggested that a borrower who pays some of his debt and then loses the receipt for his payment must pay off the full loan.

At the end of our daf, we learn that predated loan documents are not valid.  The rabbis are concerned that people could use their receipts to 'prove' that their claims are true when in fact they are seeking improper financial gain.