Gemara

83b (Mishnah): If Reuven sold (what was understood to be) good wheat, and it was found to be bad, the buyer (Shimon) can retract. If he sold bad wheat, and it was found to be good, Reuven can retract. If he sold red wheat, and it was found to be white, either can retract.

92a (Mishnah): If Reuven bought Shimon's Peros and planted them, and they did not grow, even if he bought flax seeds, Shimon is exempt;

R. Shimon ben Gamliel says, if he bought seeds that are not (normally) eaten, Shimon is liable.

93b: They argue only about how much Shimon pays. Chachamim obligate only the value of the seeds R. Shimon obligates also the expenditures.

93a (Beraisa): If Reuven bought Shimon's fruit and planted it, and it did not grow, if he bought seeds that are not eaten, Shimon is liable. If he bought flax seeds, Shimon is exempt;

R. Yosi says, Shimon must return the value of flax seeds for planting.

Chachamim: Many people buy flax seeds for other things!

Suggestion: R. Yosi and Chachamim who responded to him argue about whether or not we follow the Rov (majority).

Rejection: Both follow the Rov. R. Yosi follows the Rov of seed bought, and Chachamim follow the Rov of people who buy.

Answer: The first Tana (does not follow the Rov). R. Yosi or the latter Chachamim do.

Bava Metzia 105b (Mishnah): If Reuven rented Shimon's field and the crop was stricken by locusts or withered (by the sun or wind), he deducts from the rental only if the entire province was stricken.

106b (Reish Lakish): He deducts only if it sprouted and was afflicted. If it never sprouted, he should have planted again. Perhaps he planted too early!

Beitzah 7a: David requested Pachya eggs (laid from a live hen). Yosef gave to him eggs found in a slaughtered hen. R. Ami ruled that it is a Mekach Ta'os.

One might have thought that David wants to eat them, and he requested Pachya eggs because they are fully developed, and Yosef must return only the difference in value. R. Ami teaches that this is not so.

Rishonim

Rif and Rosh (6:1): He is liable only if the seeds did not grow due to themselves. If something else caused them not to grow, e.g. hail, he is exempt.

Question (Rosh): The Rif connotes that if we do not know the reason why did not grow, e.g. hail, we assume that it is due to the seeds. This is difficult, for Reish Lakish taught that a rented cannot deduct If it never sprouted, for he should have planted again; perhaps he planted too early. We do not say that it did not sprout due to his seeds!

Answer (Rosh): Really, we attribute the problem to the seeds. However, if the entire valley was stricken, surely, not everyone had bad seeds! For one person, we assume that he had bad seeds; one opinion relies on this to obligate paying the expenditures!

Rambam (Hilchos Mechirah 16:1): If Shimon sold seeds that are not eaten, and they did not grow, he is liable. He returns the money paid for them, for the Chazakah is that they were bought for planting. This is if they did not grow due to themselves. If the land was afflicted, e.g. through hail, he is exempt, for perhaps the hail caused that they did not grow. The same applies to all similar cases.

Ramban (103b DH v'Havi): Since Pachya eggs are more important because they are fully developed, this is no worse than a mistake in the weight. Even so, (had David wanted to eat them) it would suffice to return the difference, for eggs are like Peros, and he can return an egg to pay the difference.

Rebuttal (Ran Kidushin 17b DH ud'Amrei): The Ro'oh's text says that he put the eggs under a hen and they did not hatch. If we would say that David wanted to eat them, Yosef would need to return only the difference, because the eggs spoiled and David cannot return them. However, if David could return the eggs, he could claim Mekach Ta'os. Many texts do not say that he put the eggs under a hen, but still we must say that he did, for if not, he could return them. A Mishnah teaches that if one sold good, and it was found to be bad, the buyer can retract. He need not accept bad, even twice as much!

Mordechai (Bava Metzia 291): Reuven claims that Shimon sold him a golden item and said that it is gold, and he broke it and found that it is of tin. Shimon says 'had you returned it to me intact, I would have accepted it and resold it. Now that you broke it, it is worthless to me. You were negligent with my item!' Rabbeinu Meir ruled that this is not negligence, for Reuven did not know that there is tin inside. Had he known, he would have returned it intact! This is like the overseer of orphans who bought an ox for them. He gave it to a cattle herder (Levi). It did not have teeth, so it died. Even though the seller bought and sold animals and did not know that it had no teeth, he was obligated to return the money (Bava Metzia 42a), for the buyer was not negligent. Therefore, Shimon returns the money. However, if Shimon does not believe that the inside was tin, he swears that he does not know that there was tin inside, and he is exempt.

Poskim

Rema (CM 232:18): If the buyer was not negligent at all, e.g. he bought a gold ring, broke it and found tin inside, the seller must return the money, even if the seller overpaid (when he bought it). If the seller does not believe that the inside was tin, he swears that he does not know about this, and he is exempt.

SMA (43): We do not obligate an oath due to a Safek claim! Why should we assume that the seller knew? Also he bought it assuming that it is gold! Perhaps this is when the buyer claims that the seller told a smith to put tin inside.

Shach (13): Since the buyer is sure that tin was inside, he holds that the seller surely owes him. The seller is unsure whether he owes him, so he swears (CM 75:9).

Taz (Teshuvah b'Sof Siman 75): If one paid a loan, and afterwards the lender says that one of them coins was Pasul, the borrower is exempt, for he does not know whether he owes. The Chazakah is that he paid with good coins; now, a Safek arose. This is like one who gave a bad coin. He must exchange if only if he recognizes it. The Mordechai says that the seller of the gold ring swears that he does not know, and he is exempt.

Rebuttal (Pischei Teshuvah 75:27, citing Panim Me'iros 1:60): If one does not recognize the coin that he gave, he does not know that he ever owed. A borrower knows that he owed, so he needs to be sure that he paid!

Shulchan Aruch (20): If Shimon sold garden seeds that are not eaten, and Reuven planted them and they did not grow, Shimon is liable. He returns the money paid for them, for the Chazakah is that they were bought for planting. This is if they did not grow due to themselves. If the land was afflicted through hail or similar things, he is exempt, for perhaps the hail caused that they did not grow. The same applies to all similar cases.

Beis Yosef (DH u'Mah she'Chosav v'Ha): The Ran (92a DH Masnisin) says that Shimon is liable only if they were sown in tested (fertile) land and in normal years. If he planted in Trashim (rocky land, or weak, fine soil), or the years were substandard (for crops), he is exempt. We attribute the lack of growth to the year or the soil, unless experts saw that the seeds were not proper for seeding. It seems that the Rif and Rambam agree.

Beis Yosef (DH u'Mah she'Chosav v'Chol, citing the Ran): What is the Chidush regarding inedible seeds? Surely they were sold for planting! The Chidush is, even though they were planted and cannot be returned, the seller returns the money, for it is as if he told the buyer to plant them. One might have thought that even though it is a Mekach Ta'os, he can say 'return the seeds, and then I will return your money.'

SMA (46): Even if the seller did not know that they are bad for planting, he should have realized that perhaps they are bad and the seeds will be ruined. This is why he must return all the money, even though the seeds rotted in the ground.