Bava Metzia, page 12 side b, page 13 side a, and page 16. Rambam, Laws of Loans, chapter 23, law 5. Shulchan Aruch, Choshen Mishpat ch 39 law 13, ch 65

The RI MIGASH and Rabainu Chananel (as brought in the Mשgid Mishna)and the Rif (as brought in the Gra)hold if there is a document testifying that there was (or there is?) a loan [שטר אקנייתא], that the lender has to make the loan, if he has not done so yet. The Rashba disagrees. Also, the Rif holds a document testifying there was a loan, it has no validity until it reaches the hands of the lender (like rav asi page bava metzia 13a), and the Rashba disagrees. Now according to Shmuel, the law is that if one finds a document in the street testifying there was a loan one must return it to the lender. The Rif (bavametzia page 16, pg 9 in pages of the rif) disagrees. To Rav Shach, the kinyan sudar [(acquisition by means of exchange of a handkerchief)] is not kinyan kesef (acquisition by means of exchange of money). However, the Rashba and holds there is no obligation to make the loan, just like the borrower can also say at the last minute that he does not need the money and does not want the loan. But he also holds the documents valid even before it reaches the lender because he decided the law like Abyee (pg 13 a) that the witnesses make the document valid the minute they sign or testify verbally. However, the fact that the document is valid does not make an obligation to make the loan since it is a kinyan sudat, not kinyan kesef. Rav Shach is trying to get the opinion of the Rashba to agree with the law of Shmuel (that one needs to return a document of a loan to the lender). . One has to return the document to the lender because the Rashba is going like rabbainu izzhak that the law is like Abayee that once the document of the loan is signed by witnesses, it is automatically valid even before it gets to the lender. hpwever The Rif disagrees with this. He holds the law is not like abyee, but like rav asi that the document needs to get to the lender before it can be valid. So the Rif is self consistant. The document of a loan is not valid until it gets into the hands of the lender, and if it found in the street, it is not returned to the lender nor borrower. While to the Rif, the lender has to make the loan once there is a kinyan sudar, we do not assume that he did so. Rav Shach wants to explain the Rashba. The Rashba holds even when there is a valid document, there is no obligation on the lender to make the loan. Yet, if the document is found in the street, it is returned to the lender as per Shmuel. And the Ramban and Rashba hold the witnesses on the document make the loan valid even before it reaches the hands of the lender unless there was a condition otherwise. This would all be impossible to understand at first glance. If we do not know from whom the document fell, then why give it to the lender? Rav Shach has already proven that a acquisition by a handkerchief, kinyan sudar, is not equal to acquisition by money, kinyan kesef, and as long as we do not know money changed hands, there is no reason for the borrower to pay back a non existent loan. The answer to all this is simple. One must ask what is written in the document of the loan? Does it say “There has been made a loan?”, or that “There is at present a loan that is being made,”? then obviously with valid witnesses, it must be paid back just as Rav Shach points out that the fall of the doc in the street is not a reason to invalidate the witnesses, only the document. But if the document says, “There will be a loan,” then in fact, there is no reason to return the document to the lender since we have no way of knowing if the loan was completed.