Siman 57

It is forbidden to keep a paid document within one's house, as it is said: "Let not unrighteousness dwell in your tents." And likewise, if he forgave the borrower his debt, it is forbidden for him to keep his document. And if he does not wish to return it, they compel him until he returns it or tears it in the presence of witnesses, if the borrower is not particular about the use of the paper being returned to him. The lender may not withhold the document because of the scribe's fee that the borrower has not yet paid. And if he wishes to write upon the document that it is paid, it is permitted [Shach]. And in a place where the borrower does not pay the scribe's fee, he must tear the document and does not need to return it to the borrower since the paper is his. And even with the consent of the borrower, it is forbidden for him to keep a paid document, for perhaps their inclination will overcome them and they will perform some conspiracy with it, such as to collect from purchasers and the like [Netivot HaMishpat]. However, it is permitted to keep a document of a maneh when he paid him fifty, when he writes a receipt upon the document or writes him a separate voucher and delivers it to the borrower. And if the time for payment of the entire document has already arrived, he may keep it even without writing a voucher so that the borrower will be forced to pay it [ibid.]. And regarding the prohibition to keep a paid document, there is no difference between a document with witnesses and a document in the borrower's own handwriting [it appears to me].

A document with which one borrowed and repaid, he may not borrow with it again, as has been explained in siman 48. And if the borrower said, "I repaid you," and the lender replied, "It is true that you repaid me, but I returned them to you," he has admitted that its lien was forgiven and he may not borrow with it again. And if he did borrow with it, he may not collect from purchasers, but from the borrower himself he may collect with it when he admits that he borrowed upon it once more. And if the borrower denies it, he cannot extract from him with this document even though he has a mingo that if he wanted he could have said "I was not repaid," for since in any case he admitted that this document is repaid, it is like a shard. Its law is only like a claim by word of mouth, where the borrower swears a hesset and is exempt. Nevertheless, if the lender seized [property], even with witnesses such that he does not have a mingo that if he wanted he could have said "I did not seize," we do not extract it from him [Shach], for the reason the lender is not believed with his mingo is because it is a mingo to extract [Tosafot Bava Batra 32b] from the borrower, and we do not say a mingo to extract, as has been explained in siman 82. Therefore, if the lender seized it and does not need now to extract from him, he is believed based on his mingo. However, according to the poskim who hold that we do say a mingo to extract, and the reason the lender is not believed with his mingo is because since according to his words in any case the document is like a shard, it is impossible to make it a loan by document, it appears that seizure with witnesses does not avail unless he seized without witnesses such that he has a mingo of "I did not seize." However, some say that even according to the poskim who say a mingo to extract, seizure with witnesses avails here [Shach], and it requires further study for the law. And know that all this is only when the lender admits that he was paid for this debt and returned it to him as another loan; but if he says that he returned the money to him because they were not good for spending except with difficulty, or even if some of them were not good and therefore he returned all the money to him, the document remains in its strength and he may collect with it even from purchasers.