Gemara

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 he is eager to pay, lest he lose the opportunity!

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.

We apply the Chazakah even against orphans. (If a man died before the time to pay his debt, the lender collects from the orphans without swearing.)

Even though (normally) one collects from orphans only through an oath, here is different, for the Chazakah is that one does not pay before the time.

158a (Mishnah - Beis Hillel): If a house fell on Reuven and his wife, and Reuven's heirs say that she died first (so Reuven inherited all her property), and her heirs say that he died first (so they inherit her property and her Kesuvah), we leave the property in its Chazakah.

158b (R. Elazar): Her heirs are Muchzak.

174a: (Rav Papa): If Levi was an Arev for Shimon, and Shimon died, and Levi paid the lender before informing Shimon's orphans, the orphans now owe Levi. Paying a creditor is a Mitzvah. The orphans are not obligated in Mitzvos (until they become adults).

(Rav Huna brei d'Rav Yehoshua): The orphans do not pay (until they mature). We are concerned for Tzrari (perhaps their father gave money to Reuven, in order that Reuven will not ask the Arev to pay).

They argue in a case when Shimon admitted just before he died that he did not pay, or he was excommunicated for refusal to pay. (There is no concern for Tzrari.)

Gitin 34b (Mishnah): A widow collects her Kesuvah from orphans' property only through an oath. Judges refrained from administering the oath. R. Gamliel enacted that she vow like the orphans desire, and then she collects.

35a: A widow (who came to collect her Kesuvah) said 'Rav does not collect a Kesuvah for a widow, lest she already received part of it. I swear by Hash-m's name that I have not benefited from my Kesuvah!'

Rav Huna: Rav admits that if she swears on her own, she receives her Kesuvah.

Rishonim

Rif and Rosh (1:9): The Halachah follows Reish Lakish, even against orphans. Normally, one does not collect from orphans without swearing. This is an exception, for one does not pay before the time.

Rosh: This is even from minor orphans. Just like one does not pay before the time, one does not give Tzrari before the time. R. Yonah says that even though one does not pay before the time, we are concerned for Tzrari before the time. People often give a security before the time, sometimes even at the time of the loan. This is why he (Rav Huna) did not explain that we do not collect from orphans due to concern lest the father already paid, rather, due to concern for Tzrari, for this applies even within the time. I disagree, for it is not common at all to give Tzrari before the time. Even Abaye and Rava say that one is apt to pay early if he gets the money, only due to fear lest he spend the money. He has no reason to give one of his articles before the time. (Note: Rashi explains that Tzrari is coins. The Rosh explains that it is an object. I did not find anyone else who says so.) If the creditor will demand payment in the time, he will give one of his articles then! We are concerned for Tzrari regarding orphans after the time. When a creditor demands payment in the time, he often grants more time if a security is given, for then he is sure he will collect. Therefore, one does not collect from orphans until they mature, for perhaps they will find witnesses that their father gave Tzrari. The reason is not concern for payment. This concern suffices only to obligate an oath, but not to delay payment until they mature, for had he paid, the creditor would have returned the document. We are not concerned lest witnesses come and say that he paid and left the document with the creditor, for this is unusual.

Hagahos Ashri: We collect from minors only to save paying Ribis or a widow's food. For them, we are more concerned for Tzrari. Even if testimony was accepted in their father's lifetime, one may not collect from them until they mature.

Question: If one may collect from minor orphans, the Gemara should have said 'even though we said that we collect from (minor) orphans only to pay a loan (from a Nochri) on Ribis (within the time, we collect any loan)', for that refers to minors. Why did the Gemara say 'even though one collects from orphans only through an oath...', which discusses adults?

Answer #1 (Rosh): Had we said 'even though we said that we collect from orphans only a loan on Ribis', one might have thought that (within the time) one collects only through an oath, like the usual law of collection from orphans. Therefore, it said 'even though one collects from orphans only through an oath...' to teach that here he need not swear.

Answer #2 (Tosfos 5b DH v'Afilu): Only Rav Huna holds that one collects within the time from minor orphans. The Gemara teaches that all (i.e. even Rav Papa) agree that one does not collect from orphans without swearing, unless it is within the time. Bava Basra 174 did not list collection from minor orphans within the time among differences between Rav Papa and Rav Huna, for they argue with Reish Lakish. Also, the Gemara does not always list all the differences.

Question: Regarding a widow, we are concerned for Tzrari even within the time! This is why she must swear to the orphans (Gitin 34b), even though it is not due to be paid in her husband's lifetime!

Answers (Rosh): Since there is a Tanai Beis Din to pay a Kesuvah, (one is zealous to ensure that he fulfills it, so) we are concerned. This is why we are concerned for Tzrari for food for one's daughter after he dies, but not for food that he obligated himself to give to step-daughters. The Ritzva says that regarding a widow, we are not concerned for Tzrari within the time. Rather, when one is about to die, it is common to give Tzrari, lest his widow be disgraced in Beis Din. This explains the opinion (158b) that when we are unsure whether Reuven or his wife died first, her heirs are Muchzak (and inherit her Kesuvah). A widow collects only through an oath, and one does not inherit (rights to collect through) an oath! According to the Ritzva, since he did not expect to die, there is no concern for Tzrari. Therefore, no oath is needed.

Rambam (Hilchos Malveh 14:1): One who collects from a minor or adult heir collects only through an oath like a mid'Oraisa oath. 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.

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. This is even from orphans. If Shimon died within the time and left orphans, even minors, Reuven collects without an oath.

Beis Yosef (DH umi'Divrei): Tosfos (5b DH v'Afilu) says that we discuss collecting only from adult orphans, but not from minor orphans, even if testimony was accepted in their father's lifetime. It seems that R. Yonah agrees. The Rosh says that it is even for minors, for there is no concern for Tzrari. The Magid Mishneh says that the Ramban and Rashba agree, and it seems that the Rambam agrees. The Ran and Sefer ha'Terumos agree. Since so many hold like the Rambam, we rule like them.