Gemara

(Rav Kahana): One may not keep in his house a document of Amanah (the borrower trusted the prospective lender and gave to him the document before the loan was given) - "Do not let Avlah (fraud) dwell in your home."

(R. Yehoshua ben Levi): One may not keep in his house a document for a loan that was paid - "Do not let Avlah dwell in your home".

(Rav): "If there is sin in your hand, distance it" refers to a document of Amanah or Pasim (a document which was written only to make the alleged lender appear rich). "Do not let Avlah dwell in your home" refers to a paid document.

R. Yehoshua forbids keeping a paid document, all the more so he forbids keeping a document of Amanah;

Rav Kahana forbids keeping a document of Amanah. He permits keeping a paid document. Sometimes the lender keeps it until the borrower pays the cost of the document.

Shabbos 78b (Beraisa): If one was Motzi a paid loan document, he is exempt;

R. Yehudah obligates.

(Rav Yosef): They argue about whether or not one may keep a paid document. Chachamim forbid, and R. Yehudah permits.

(Abaye): All forbid keeping a paid document. They argue about whether a lender must validate a document when the borrower agrees that he authorized it.

Bava Metzia 16b (Rava): When a loan document is found, we should be concerned for payment. We do not say that the borrower caused his own loss by not tearing the document. Perhaps the lender promised to return the document at his next opportunity, or was holding it until the borrower paid the scribe's fee!

18a - Suggestion: Perhaps one is believed to say that he paid an obligation that Chachamim enacted!

Rejection: If so, in a place where they do not write Kesuvos, why does a woman collect a Kesuvah by showing her Get? He could claim that he paid!

Suggestion: Perhaps that is no claim, for then he should have torn the Get (to stop her from collecting again).

Rejection: He cannot tear it, for she needs it to remarry!

68a: In documents of Chuznai, wholesalers who sell goods to merchants write in the document the principal and half the anticipated profit. This is forbidden, lest there will be no profit (and the borrower will pay Ribis)!

Mar bar Ameimar: My father does so. He believes a buyer who says that he did not profit.

Rav Ashi: He should be concerned lest he die, and his heirs will not trust the buyer!

Rishonim

Rif: One may not keep in his house a document of Amanah or a paid document.

Ran (DH l'Olam): Letter of the law, witnesses may sign on Amanah. However, since the lender may not keep it, they should not do so.

Ran (DH Asur): The borrower should pay the cost of the document. Sometimes he has no money at the time, and the lender pays it, to be repaid when the loan is repaid. The Gemara proves that one may not keep a paid document to be repaid its cost.

Drishah (CM 57:1): Bava Metzia 16b connotes otherwise. What is the Ran's proof? Surely all agree that it is Avlah to keep the document after the borrower paid for its cost. We must say that they argue about whether this is forbidden even before this, lest one keep it afterwards.

Ran (DH Tenai): If a document is on condition, one may keep it in his house because some lien begins now. This is no worse than keeping a document for 100 even though only 50 is owed. One opinion allows even to keep a paid document in order to be paid the cost of the document.

Rosh (2:11): Rav Kahana forbids keeping a document of Amanah. He holds that the verse would not call keeping a paid document Avlah, for sometimes one may keep it, i.e. until the borrower pays its cost. R. Yehoshua holds that one should be careful to not to keep it after its cost was paid. If he kept it, this is called Avlah Keeping it to be paid its cost is not called Avlah. It is not improper!

Note: R. Ami expounded that one may not keep an uncorrected Sefer for more than 30 days from our verse. The Ritva (DH mi'Kan) says that less than 30 days is not called "Tashken" (dwelling). I did not find anyone give this Shi'ur regarding keeping a paid document.

Poskim

Shulchan Aruch (CM 57:1): One may not keep in his house a paid document (Rema - or a document she'Nimchal Shibudo (its lien was pardoned)).

SMA (2): She'Nimchal Shibudo is when the lender pardoned his lien on land, or when the loan was paid, even he borrowed again with the same document.

Rebuttal (Shach 2): One may keep a document even if he pardoned the lien on land, since it is valid to collect from Bnei Chorin. This is no worse than keeping a document for 100 even though only 50 is owed.

Shulchan Aruch (ibid.): If one does not agree to return a paid document, it is proper to excommunicate him until he returns it.

Source (Rashbatz 1:24, cited in Beis Yosef DH Asur): We excommunicate for transgressing "Do not let Avlah dwell in your home".

Shach (61:16): This is even if the borrower does not demand the document back. One does not transgress if he gave a receipt. Alternatively, perhaps even then one transgresses (unless the receipt is written on the document), for there is no reason to keep the document. The receipt might get lost!

Shulchan Aruch (ibid.): One may not keep the document to be paid its cost.

Question (Drishah 1): Why is there no mention of the opinion of Tosfos, the Rosh and Ritva (who permit this) in the Shulchan Aruch or Beis Yosef? Why didn't the Ran mention that Tosfos argues?

Note: It seems that the Drishah never saw Bedek ha'Bayis (additions that the Beis Yosef later added to and incorporated in his Sefer), which cites them.

Question (Shach): Why does the Shulchan Aruch rule like the Ran?! It seems that if one tears the document or writes on it that it was paid, all permit keeping it. It seems that the Shulchan Aruch disagrees.

Answer (Gra 3): Rav Kahana does not call keeping a paid document Avlah, for sometimes the lender keeps it to be paid its cost. This connotes that R. Yehoshua forbids even this.

Rema: In a place where the borrower does not pay the cost of the document one need not return it, as long as he tears it.

SMA (5): If the borrower paid for the document, even if this is not the custom in that area, one must return it for it is the borrower's.

Gra (4): In Bava Metzia 18a, we suggested that the husband should have torn her Get when he paid the Kesuvah. We did not suggest that he take it back, because she pays for it.

Shach (5): The Rema explains that the Shulchan Aruch discusses who is entitled to keep the document (e.g. to cork a bottle) even after it is torn. Ir Shushan says so explicitly. This is wrong. (That is discussed in 61:12). Here the Shulchan Aruch discusses the Isur to keep an intact paid document.

Shulchan Aruch: One may keep a document for 100 even if only 50 is owed, if a receipt is given to the borrower or written on the document.

Rebuttal (Shach 6): The Shulchan Aruch explains that the Ran permits to keep such a document only when he wrote a receipt. This is wrong. If so, also keeping a document to be paid its cost is when a receipt is written on it. If so, why does anyone forbid?! Whenever David holds Levi's property, he may retain it for a debt Levi owes him for an unrelated reason. Avlah is only when one keeps a document for no proper reason. Not writing a receipt is not Avlah. Many people trust each other!

Defense (Korban Nesan'el (on Rosh), 4): In Bava Metzia (68a), we are concerned lest a lender die and his heirs will collect improperly with his document. If there is no receipt, the same applies here! The Heter to keep the document to be paid its cost is when a receipt is given to the borrower (the cost of the document is deducted), or written on the document. Or, he tears the document (unknown to the borrower, lest the borrower feel no need to pay). It seems that the Rosh and Ran do not argue. The Ran forbids keeping a paid document without tearing it or writing a receipt, and the Rosh permits when one tears it or writes a receipt.