Gemara

(Mishnah): We are not concerned lest a weasel dragged Chametz from one house or place [that was not checked] to another house or place [that was checked].

9b - Question: The coming Mishnah is concerned!

(Mishnah): Chametz that one leaves over [after the Bedikah, to eat before the time of Bi'ur] must be put in a covert place, so one will not need to check again [lest weasels dragged it elsewhere].

Answer (Rav Mari): The latter Mishnah is a decree lest one leave 10 pieces of Chametz and later find only nine.

10a: If one left nine pieces of Chametz and later found 10, this is like the argument of Rebbi and Chachamim;

(Beraisa - Rebbi): If one left 100 Zuz [of Ma'aser Sheni] and later found 200, we consider it to be a mixture of Chulin and Ma'aser;

Chachamim say, it is all Chulin. (We assume that the original money was taken, and this is new money.)

If one left 10 pieces of Chametz and later found nine, this is like the Seifa;

(Beraisa - Rebbi): If one left 200 Zuz [of Ma'aser] and later found 100, it is from the initial money, and the other 100 was taken;

Chachamim say, it is all Chulin.

Beitzah 10b (Mishnah): If one designated three birds [to slaughter them on Yom Tov], and found two, they are permitted.

We say that these are from the initial three, and one went away.

Suggestion: This is like Rebbi, and unlike Chachamim!

Rebuttal: It is even like Rabanan. R. Yochanan and R. Elazar taught that chicks are different, for they are prone to walk.

Question: Why did both of them need to say so? R. Yochanan and R. Elazar argued about the Mishnah of Ma'aser. One says that the Tana'im argue about [when all 200 were in] two wallets, but if there was one wallet, all agree that the money is Chulin. The other says that the Tana'im argue about one wallet, but if there were two wallets, all agree that the initial money is there;

The latter opinion did not need to say that chicks are different. Here is like two wallets. He holds that all agree in such a case!

Answer (Rav Ashi): We discuss tied wallets and chicks. Chicks can become untied. Wallets cannot become untied [by themselves].

Rishonim

Rambam (Hilchos Chametz 2:7): If one checked on the night of the 14th and left 10 loaves [of Chametz] and found nine, he must check again, for surely a weasel or mouse dragged [a loaf].

Rambam (12): If one left nine loaves and found 10, he must check [again].

Rambam (Hilchos Ma'aser Sheni 6:3): If one left 100 Zuz of Ma'aser Sheni and he found 200, or he left 200 and he found 100, even if they are in two wallets, it is all Chulin.

Rebuttal (Ra'avad): The Mishnah says that it is all Ma'aser.

Kesef Mishneh: The Rambam learns from Beitzah, Pesachim and the Yerushalmi, which say that the Mishnah is Rebbi, and Chachamim disagree. The Halachah follows them, for they are the Rabim. Even though in Yevamos 42b, we say that we follow a Stam Mishnah even if there is an argument in a Beraisa, here is different, for in Beitzah Amora'im toiled to establish the Mishnah like Chachamim. This shows that the Halachah follows them, against the Stam Mishnah.

Tosfos (10a DH Heni'ach): The case is, all the loaves are tied together, like the argument of Rebbi and Rabanan. In Beitzah, we say that they argue about tied wallets. If they are not tied, all agree that Ma'aser and Chulin are mixed together. Here, we cannot attribute [the movement of money] to mice, but we attribute it to minors.

Poskim

Shulchan Aruch (OC 439:3): If one left nine loaves of Chametz and found 10, he must check to find all nine.

Beis Yosef (DH Heni'ach): Rashi says that Chachamim hold that the first money was taken, and this is new money. Similarly, the loaves he left are not here now. He must check the entire house, lest weasels dragged them. Someone else brought the 10 found. The Halachah follows Chachamim.

Beis Yosef (DH u'Mah she'Chasav Rabeinu): The Tur says that this is even if he was Mevatel. It seems that the Rambam agrees. This is because it is as if there is no Safek at all. If not, Chachamim would not be lenient to say that it is all Chulin. Therefore, even if he did Bitul, he must check.

Mishnah Berurah (12): We say that these are other [loaves] because we do not recognize them.

Shulchan Aruch (ibid.): Similarly, if one left 10 and found nine, he must check for all 10, for we say that what he left was taken, and these are other loaves.

Pri Chodosh: The Bach (Sof ha'Siman) asked why the Rambam omitted the law of one who left 10 and found nine. I say that since he taught that one who left nine and found 10 must check, all the more so one who left 10 and found nine must check! It seems that the Bach overlooked what the Rambam wrote in Halachah 7 "if one left 10 and found nine he must check again, for surely a weasel or mouse dragged." However, the Rambam did not explicitly say that he must search for all 10. I say that we can learn from one who left nine and found 10. He must search [for all nine]. Also "he must check" connotes the entire house.

Shulchan Aruch (ibid.): Some say that this is only if they were tied together. If they were not tied, he needs to search only for the one.

Mishnah Berurah (14): This refers to the latter case (he left 10 and found nine).

Beis Yosef (DH u'Mah she'Chasav v'Ha): Tosfos and the Tur rule like the opinion that they argue about one wallet. However, the Rambam (in Hilchos Ma'aser) rules like the opinion that they argue about two wallets. If there is one wallet, all agree that it is all Chulin. If so, we do not establish the case to discuss tied wallets. Rather, it is even if they are not tied. If so, even if the loaves are not tied, if one left 10 and found nine he must check.

Magen Avraham (7): If he left nine and found 10, he must search for all 10.

Gra (DH v'Yesh): This opinion holds like the opinion that they argue about one wallet, since Rav Ashi establishes the Mishnah like this. The Rambam holds that since here (regarding Chametz) the Gemara did not distinguish, this shows that it holds like the opinion that they argue about two wallets.

Pri Chodosh: Many explain like Tosfos and the Tur, unlike the Rambam. The Tur (YD 63) brings like the Rambam from the Rashba (Toras ha'Bayis ha'Katzar 4:2, b'Sof. If one left 10 pieces of meat and found nine, we are concerned that they are not from the 10 he left.) This (Tosfos' opinion) is primary. On 9b, the Gemara connotes that we decree to leave the Chametz in a guarded place, lest he leave 10 and find nine. This connotes that he would have to search only for the missing loaf. We must say that that is when they are not tied. (Only when they are tied, one must search for all 10.)

Mishnah Berurah (15): Surely, whoever took them took all of them. Perhaps minors, a mouse or weasel took them.

Mishnah Berurah (16): The Acharonim rule like this.

Mishnah Berurah (17): One should be lenient like this only if he did Bitul, or there is still time to be Mevatel (i.e. before the sixth hour on Erev Pesach, when Chametz becomes Asur b'Hana'ah). When he left nine and found 10, it makes no difference whether or not he left tied loaves and found tied loaves, or left separate loaves and found an addition one. In every case he must check for all nine that he left.

Shulchan Aruch (497:13): If one designated three birds [before Yom Tov, to slaughter on Yom Tov] and found two, they are permitted.

Rema: Even if they were tied together, they can become untied.

Gra (DH v'Afilu): This is like Tosfos and the Tur, who say that they argue about one wallet.