Avi Ezri, laws of loans 23 law 5

I was on my way back from the sea shore and it occurred to me that there is a slight problem in the answer of Rav Shach for the Rashba (in the Avi Ezri, laws of loans 23 law 5). The Rashba holds after a valid document of a loan is written and singed there still is no obligation to give or receive the loan since it is only aacquisition by handkerchief, kinyan sudar. There is no loan until money has been exchanged. Yet the Rashba follows the decision of Rabainu Izhack that we follow Abyee that the witnesses on the document already cause the acquisition to be valid. “Witnesses by their signature cause acquisition," and that witnesses on the document are considered as if their testimony was already tested and verified in court. And the Rashba holds the law is like Shmuel that when document of a loan is found in the street, it is returned to the lender. The problem with this decision is that if there is no obligation to go through with a loan even after there is exchange of a handkerchief, kinyan sudar, and a valid document, and we do not know from whom the document fell, maybe it fell from the borrower? Maybe he paid it back, and it fell from him on his way home? Rav Shach answers this by the fact that the fall of the document into the street causes a doubt in the validity of the document, but not in the validity of the witnesses. The problem I see in this answer is that once you say there is a problem in the document, then there is automatically a problem with the witnesses that have signed the document. After all, we do not have separate witnesses. The only witnesses we have are the ones that have signed the document.------------------------------------------I was on my way back from the sea shore and it occurred to me that there is a slight problem in the answer of רב שך for the רשב’’א (in the אבי עזרי, laws of loans 23 law 5). The רשב’’א holds after a valid document of a loan is written and singed there still is no obligation to give or receive the loan since it is only acquisition by handkerchief, קנין סודר. There is no loan until money has been exchanged. Yet the רשב’’א follows the decision of רבינו יצחק that we follow אביי that the witnesses on the document already cause the acquisition to be valid. “עדים בחתימתם מזכים לו," and that witnesses on the document are considered as if their testimony was already tested and verified in court. And the רשב’’א holds the law is like שמואל that when document of a loan is found in the street, it is returned to the lender. The problem with this decision is that if there is no obligation to go through with a loan even after there is exchange of a handkerchief, קנין סודר, and a valid document, and we do not know from whom the document fell, maybe it fell from the borrower? Maybe he paid it back, and it fell from him on his way home? רב שך answers this by the fact that the fall of the document into the street causes a doubt in the validity of the document, but not in the validity of the witnesses. The problem I see in this answer is that once you say there is a problem in the document, then there is automatically a problem with the witnesses that have signed the document. After all, we do not have separate witnesses. The only witnesses we have are the ones that have signed the document.