Gemara

4a - Question: If one rented a house on the 14th b'Chezkas Baduk (on the pretense that it was checked), and it was found to be unchecked, is this a 'Mekach Ta'os' (a mistaken rental, and the tenant may retract)?

Answer (Abaye): The rental is valid where people themselves check for Chametz, and even in a place where they hire others to check, for one is happy to do a Mitzvah with his property.

10b Question (Rava): If a loaf is in a snake's mouth, must one bring a snake charmer to remove it, or not?

Perhaps Chachamim make a person exert himself, but they do not require him to spend money [for what he himself cannot do]. Or perhaps he must even spend money!

This is not resolved.

Bava Metzi'a 29b (Shmuel): If one found Tefilin in the market, he estimates their value and may wear them immediately. (If the owner comes and gives a sign, he gives their value to him.)

(Beraisa): One who borrowed a Sefer Torah may not lend it to someone else.

Question: Why was this taught about a Sefer Torah? It applies to any borrowed object!

Answer: One might have thought that a Sefer Torah is different, for one is happy to do a Mitzvah with his property. The Beraisa teaches that this is not so.

Rishonim

Rambam (Hilchos Chametz 2:18): If one rented out his house b'Chezkas Baduk, and it was found to be unchecked, the tenant must check. This is not a Mekach Ta'os, even where they hire others to check, for he does a Mitzvah.

Hagahos Maimoniyos (2): In Bava Metzi'a, we concluded that one is not happy to do a Mitzvah with his property! Tosfos answered that there we discuss a Mitzvah without the owner's knowledge. Surely one is happy to do a Mitzvah with his property when he knows about it.

Magid Mishneh: The Ro'oh bring from the Ramban that surely the landlord must pay the loss he caused, since he stipulated. The question is whether it is a Mekach Ta'os, and even if the landlord agrees to pay the cost of checking, the tenant may retract. Abaye concluded that he cannot retract, for one is happy to do a Mitzvah with his property, since he will be paid back. If not, we do not say [that one is happy to do a Mitzvah with his property].

Ran (2a, top of the page): We conclude that one is happy that to fulfill a Mitzvah, even with his property. He cannot claim from the landlord what he paid someone to check, for he did not benefit the landlord. The landlord merely had a Mitzvah to do. Even though he did not do it, there is no monetary obligation on him. Tosfos (note: our Tosfos does not say so) says that the landlord must pay. This is wrong.

Me'iri (4a DH ha'Maskir): One is happy to do a Mitzvah with his property, i.e. for a small wage. We do not say so about a large amount. If there is Chametz in a snake's mouth, one need not pay a snake charmer to remove it. Chachamim did not exert a person financially. One who borrowed a Sefer Torah may not lend it to another. We do not say that one is happy to do a Mitzvah with his property! Some say that this is because the owner has a Mitzvah also without the latter lending. Some say that the tenant is happy to do the Mitzvah on condition that the landlord pay back the wages of the checker.

Nimukei Yosef (Bava Basra 44b DH Amar ha'Mechaber): My Rebbi (the Ritva) permits wearing another's Talis or Tefilin without his knowledge, since one is happy to do a Mitzvah with his property. I do not understand. One who finds Tefilin may wear them only after estimating their value and buying them for himself. We do not permit due to the Mitzvah!

Poskim

Shulchan Aruch (OC 437:3): If one rented out his house b'Chezkas Baduk, and it was found to be unchecked, the tenant must check. This is not a Mekach Ta'os, even where they hire others to check, for he does a Mitzvah.

Magen Avraham (5): One is happy to do a Mitzvah with his property or body. This refers only to a monetary loss regarding the Mitzvah. One is not happy if there will be a loss after the Mitzvah. See Bechoros 58.

Mishnah Berurah (11): If he rented on the 14th, the Chazakah is that it was checked, or the landlord explicitly said that it was checked.

Mishnah Berurah (12): It is not a Mekach Ta'os, for one is happy to do a Mitzvah with his property. Even had he known, he would have bought (i.e. rented) the house. Now he screams for a different reason, i.e. he found a better house [and seeks an excuse to retract].

Birkei Yosef (5): If Reuven rented a house to Shimon on the 13th and said that it was not checked, and on the night of the 14th he told him that it was checked, and Shimon claims that he rented it only in order to check, and he cannot obligate it to be checked (e.g. he has no Chametz to put in it), the judge uses his judgment to determine whether Shimon rented it for residence, or in order to fulfill Bedikah. If it is a Safek, ha'Motzi mi'Chavero Alav ha'Re'ayah.

Rema: Some say that he must return the wage of checking, since he stipulated that it is checked.

Magen Avraham (6): This opinion holds that one is happy to do a Mitzvah with his property only without monetary loss, e.g. to bless on his Talis. In YD 291:4, why didn't the Rema mention the opinion that requires returning the money? I answer that there, there is no loss. The Mezuzah is worth the money he pays for it. (Machatzis ha'Shekel - he can force the next resident to pay him for it.)

Magen Avraham (7): In CM 230:5, the Rema rules Stam that any defect not in the sold item itself, one deducts the money [needed to fix it], and the sale stands. If so, why did the Gemara need to say that one is happy to do a Mitzvah with his property? The law is the same for all matters! I answer that there, he did not stipulate. Here, even if he stipulated, the sale stands.

Gra (DH v'Yesh): In Bava Metzi'a, we do not say that one is happy to do a Mitzvah with his property! Rather, one is happy only when there is no loss. This is the distinction between Tefilin and Seforim. (One may borrow Tefilin Stam, but not Seforim.) The Magid Mishneh and Rema say [that he must pay] because he stipulated. This is not relevant. Here, we cannot say that he is happy, for he stipulated! Surely, the landlord must pay. Tosfos, the Mordechai and Hagahos Maimoniyos already answered that one is happy to do a Mitzvah with his property only with his Da'as. See Bechoros (58a (Damesek Eliezer - this should say 18b).) The Magid Mishneh initially said "since he stipulated", but he concludes that Abaye answered that one is happy, since he will be paid back. If not, we do not say "one is happy", even Stam (without a stipulation). He uses this to answer the question from Bava Metzi'a. The Mechaber holds like the Ran, that he is not paid back.

Gra (DH Ho'il): This opinion holds that we must say that he stipulated, for if not, there is no Hava Amina to say that there is a Mekach Ta'os, since the defect is not in the sale item itself.

Mishnah Berurah (14): [He is paid back] only where people hire others to check.

Mishnah Berurah (15): The Poskim rule like the first opinion. The landlord can say "I know that the Halachah follows the first opinion." Ha'Motzi mi'Chavero Alav ha'Re'ayah.

Bi'ur Halachah (DH v'Yesh): The Pri Chodosh says that the Mechaber and Rema do not discuss one who explicitly stipulated 'on condition that it is checked.' If he did, all agree that it is a Mekach Ta'os. The Rema cites the Magid Mishneh, who brought from the Ramban 'since he explicitly stipulated...' This is not literally true. Rather, the landlord said that it was checked, and presumably the tenant intended to rent a checked house. Maharam Chalavah (a Talmid of the Rashba) explicitly says so. However, the Ritva explains that the Ramban holds that it is not a Mekach Ta'os because it is not a total Kepeida (he is not so adamant about it), since it is a Mitzvah, and he will be paid back. Mekor Chayim rules like the Pri Chodosh. This requires investigation.

Kaf ha'Chayim (31): If he rented it on the 13th, all agree that he need not return the wage of checking.

Teshuvas Chasam Sofer (1:2): The Magid Mishneh brings from the Ramban that the tenant deducts from the rental the wages he paid someone to check. There is no monetary loss, just he pays [some of the rent] early. One is not happy to do a Mitzvah with his property regarding a real loss, like it says in Bava Metzi'a about a Sefer Torah and Tefilin. The Poskim learn from there to Talis (Nimukei Yosef). However, the Ran says that he does not deduct. Darchei Moshe (437:1) asked why Bedikah is different than a Talis. Tumas Yesharim (46) distinguishes between a loss at the time of the Mitzvah, and a loss afterwards, e.g. if he will not fold the Talis after using it. The Magen Avraham (437:5) alluded to this. However, the loss to a Sefer Torah is while using it! The Magen Avraham (14:10) answered that there is a big loss to a Sefer Torah, but checking is a small wage.

The Mordechai (Bava Metzi'a 263) said that one obligates himself for Bedikah, but not in general. I.e. he obligated himself to do a Mitzvah with is body, for one is happy to exert for a Mitzvah. He willingly paid someone to exempt himself of the toil.