Translation:Shulchan Aruch/Choshen Mishpat/57
Paragraph 1- One is prohibited from retaining a repaid document in his home and the same applies for a document whose lien has been waived. If a party does not want to return a repaid document, it is appropriate to place a nidui on him until he returns it. A party may not hold on to the document in order to obtain reimbursement of the cost of the scribe. In a place where the borrower does not pay for the scribe’s wages, the lender is not required to return the document so long as he tears it up. A lender may, however, hold on to a document of a maneh even though the borrower has paid back 50. The lender must write a receipt on top of the document for that which was repaid or write a separate receipt and provide it to the borrower.
Paragraph 2- A document that one borrowed with and repaid cannot be reused to borrow again, as was explained in Siman 48. If the borrower says he paid back and the lender says it is true but the lender gave the borrower the money back, the lender has admitted that the lien has been waived and he cannot collect with the document. If, however, the lender says I returned the money to you so that you could exchange it because the money would only be accepted in a pressing situation, the document is still valid.