Gemara

Bava Basra 5a (Reish Lakish): If Levi fixed a time to pay his debt to Yehudah, and claims that he paid within the time, he is not believed. We are happy if people pay on time. We do not believe that he paid early!

(Abaye and Rava): He is believed. Sometimes one gets the money and is eager to pay, lest he lose the opportunity!

(Mishnah): We assume that Shimon paid, unless Reuven can prove that he did not pay.

Question: If he says that he paid in the proper time, surely we assume that he did!

Answer: Rather, he says that he paid before he had to. This teaches that one does pay before the proper time!

Rejection: This case is different, because he cannot wait to pay. The payment is due for each row of bricks when it is built!

In practice, Rav Papa and Rav Huna brei d'Rav Yehoshua ruled like Abaye and Rava. Mar bar Rav Ashi ruled like Reish Lakish.

The Halachah follows Reish Lakish.

Rishonim

Rif and Rosh (1:9): The Halachah follows Reish Lakish.

Rosh: Reish Lakish discusses when in front of witnesses, a time was set for payment. Even though we say that a Stam loan is for 30 days (Makos 3b), we do not consider this to be within the time. People often pay within 30 days. If a time was set not in front of witnesses, the borrower would be believed to say that he paid within the time, Migo he could have said that no time was set. Or, we would learn from here that we do not apply Migo against a Chazakah! (The Gemara asked about this, and did not resolve it.)

Rosh: If one says 'I paid within the time', even though he is not believed, the creditor must swear Heses, since the borrower has a Vadai claim. Orphans do not obligate an oath due to Safek.

Hagahos Ashri: The creditor needs to swear only if the borrower demands that he swear.

Rambam (Hilchos Malveh 14:1): In the following cases, one collects only through an oath like a mid'Oraisa oath: a loan document was partially paid; one witness testifies that it was paid; he seeks to be paid in the borrower's absence; a creditor collects land bought from the borrower, or from a minor or adult heir. If the debt was for a time and he demanded payment before the time, he collects without an oath. After the time, he collects only with an oath.

Magid Mishneh: Why does he include a loan document that was partially paid? The lender admits that the Chazakah does not apply!

Question (Lechem Mishneh): The Tur (84) says that Rav Hai Gaon obligates an oath to collect a partially paid document even within the time. If so, why didn't the Gemara (Shevuos 41a) mention this difference between a partially paid document and one that is fully owed?

Answer (R. Akiva Eiger Drush v'Chidush Kesuvos 87a): The Gemara asked against Rav Papa, who holds that one pays within the time. The Gemara asked from a Beraisa, and not from a Mishnah about partial payment of a Kesuvah, for Rav Papa could agree that one does not pay within the time something he might never need to pay.

Mishneh l'Melech: If the Magid Mishneh holds that the lender should need to swear, this is good. If he holds that the borrower should be exempt regarding a Milveh Al Peh, because the Chazakah did not apply (to the part he paid), also a document should not be collected, for the Chazakah 'if you paid, why do I still have the document' does not apply (he paid part and left the document with him)! Perhaps partial payment destroys the Chazakah, but we can say that they intended to write a receipt, but failed to.

Rambam (Hilchos Sechirus 7:3): If a renter says 'I paid the rental I owe', and the landlord says that he did not receive it, whether the rental was with a document or through witnesses, if he demanded payment within 30 days, the renter must prove that he paid. The landlord swears Heses. If he demanded payment after 30 days, even on the 30th day, the landlord must bring a proof.

Hagahos Ashri (Bava Metzia 8:31): R. Tam says that the Chazakah applies only to a loan. One does not pay it early, for it was given to be spent. One may not touch a deposit, so even if it was given for a set time, he can say 'I returned it within the time.'

Mordechai (467,468): A case occurred in which Avraham hired a scribe. The scribe did part of the job, and was killed. His widow claimed wages for what he wrote. Avraham said that he paid for more than the scribe wrote. Gedolim ruled that he is believed. Even though the Halachah follows Reish Lakish, that one does not pay early, and Sechirus is paid only at the end, we say (regarding a wall) that the money for each row is due when it is completed. Here also, money for each treatise is due when it is finished. The same applies to every job divided into discrete parts; ha'Motzi mi'Chavero Alav ha'Re'ayah ha'Motzi mi'Chavero Alav ha'Re'ayah. I say that Avraham is believed due to a Migo. He could say that he promised to him only his expenses; this is often done. Also, here the Chazakah is flimsy, for one often gives to a scribe or teacher some of the wage before the Melachah is finished. R. Yehonason says that 'Sechirus is paid only at the end' applies only to something limited. Also, (perhaps) the Chazakah that one does not pay before the time is only when both of them claim Vadai. Here, the widow is unsure, and claims due to the Chazakah. The defendant is Vadai, he has Chezkas Mamon and a Migo that he stipulated to pay him before the time. Therefore, he is believed. Also, time passed after the scribe died. This is like one who claimed after the time, and the borrower said that he paid within the time, which was never settled. There is no difference if he or his heirs demands his wages. Even if the scribe demanded his wages, Avraham has a Migo, for he could say 'I stipulated to pay you within the time.' For every unsettled monetary law, ha'Motzi mi'Chavero Alav ha'Re'ayah.

Poskim

Shulchan Aruch (CM 78:1): If a time was fixed for a debt, and Reuven demanded payment before the time, and Shimon said 'I paid you', he is not believed. There is a Chazakah that one does not pay before the time.

SMA (1): This is only when he fixed a time, for then the borrower showed that he needed the money until that time.

Beis Yosef (DH v'Chosav): The Nimukei Yosef (3a DH ha'Kove'a) says that the same applies to rental, which is paid only at the end. If he said 'I paid within the time', he is not believed. Bava Metzia 102b proves this. The same applies to Pidyon ha'Ben.

SMA (2): The Mordechai and Darchei Moshe say that this is only when the claimant is Vadai, but not if he is unsure.

Shach (1): The Mordechai was unsure about this. He did not rely on this alone to exempt Avraham; he combined it with other reasons. It seems that the Chazakah applies even when the claimant is unsure. In a Teshuvah (brought in the Tur 71:21) the Rosh applies the Chazakah to heirs (with an unsure claim), but witnesses of payment override the Chazakah.

Rema: Similar, he cannot say 'you pardoned me.'

Rema: Some say that this Chazakah applies only to a loan, but if he had a deposit of money and uses it for their needs, it is like any deposit. He is believed to say that he returned even within the time. Sechirus is like a loan. It is paid only at the end, and one is not believed to say that he paid within the time.

Source (Darchei Moshe 2, citing Mahariyo 89): The Chazakah is only when the borrower benefits from the loan.

Shach (8): It seems that the Rosh (Teshuvah 86:1) and Tur (108:7) disagree. (They say that one is not believed to say that he returned an Iska (half-loan, half-deposit) within the time. However, perhaps there, since he is not believed to say that he returned the loan half within the time, he is not believed to say so about the deposit half. Also, there he benefits from the deposit, for there is a lien on it to compensate losses in the loan.

Shach (9): Mahara Sason says that the Chazakah does not apply to paying merchandise. Hagahos Ashri and Mahariyo connotes that it does not apply to only when one may not touch the deposit, but it applies whenever one benefits from it, e.g. paying merchandise.