It is hard to find time for extra he'aros on today's daf since the pashut p'shat is so difficult, but here is something:
The Ketzos HaChoshen (65:9) points to an interesting contradiction in Rashi regarding the problem with a shtar that has been lost being suspect to being forged or paid. Rashi on 7b in the case of a shtar that ends up in the hands of a third party, writes that it shouldn't be returned to the borrower or lender unless there are witnesses to verify that it fell from the lender. Rashi holds that the reason for not returning the contract is that we have no idea if it is a valid contract and/or if it has already been collected. But, if we see the contract fall from the lender we would return it to him which would give him the ability to collect with it. But in our gemara we say that when the shtar is lost there is an automatic "rei'usa" in the shtar, so how can we return it to the lender to collect with? Rashi must be holding that we would normally be able to assume that the fact that the contract wasn't destroyed, it must have fallen from the lender. However, since it was lost it creates a rei'usa that forces us to be concerned that it fell from the borrower - therefore, if we have witnesses that it fell from the lender we can return it him.
However, Rashi 12b seems to say that when a shtar is lost it is a rei'usa in the entire validity of the contract based on the logic that if it would be valid he wouldn't have lost it. This would force us to suspect that even if we knew it fell from the lender we shouldn't return it to him. Based on this approach, if the lender would find it himself he can collect with it, but once it fell from him we would never return it to him even if we know through witnesses that it fell from him.
Tosafos 13b asks, why can't we return a contract that falls to the lender even if the borrower claims that it is forged - since the lender can't collect without being mekayem the signatures, if he manages to do so that will prove the validity of the contract? Tosafos answers that since the borrower is claiming that it was forged and their is a rei'usa that it fell, we don't trust the kiyum of the signatures because he may have done such a good forgery job that he fools the witnesses into thinking that it is actually their signatures. However, Tosafos is not sure about using the contract to collect from the borrower himself - maybe the kiyum would allow the lender to collect from the borrower himself because he has the ability to bring witnesses to contradict the witnesses of the lender. In the process of the discussion, Tosafos writes:
אפילו יקיימנו יש לחוש שמא מזוייף הוא ולכך קאמר לא יחזיר פן יטרוף כשתשתכח הנפילה
This would imply that so long as we know that it fell, even if it somehow ends up in the hand of the lender, he would not be allowed to collect using this contract. If it is assur for the lender to collect using this contract, we would certainly not return it to him even if there are witnesses that it dropped from him (which is against rashi on 7b).