Rambam - 1 Chapter a Day

The Maggid Mishneh and the Shulchan Aruch (Choshen Mishpat 233:1) explain that this law applies even if the difference in value between the two is less than a sixth. Although the laws of ona’ah do not apply, the wheat is not of the quality specified by the seller, the transaction is considered to have taken place under an erroneous premise, and the purchaser has the right to nullify it.
From the fact that the seller cannot retract [in contrast to the instances mentioned in the following halachah], we see that the transaction is not nullified entirely, for the seller does not have the right to retract.
The distinction between the two is that the articles that are mentioned in the following halachah are clearly of different types. In this instance, although the descriptions “high quality” and “low quality” do distinguish one article from another, the distinction is not as clearly defined.

This addition is made on the basis of the gloss of the Maggid Mishneh.

In this instance as well, the Maggid Mishneh states that the principals have the right to retract even though there was no difference in value between the two types of produce.

The rationale is that both the purchaser and the seller can claim that the transaction was undertaken under an erroneous premise.

It would appear that the wine had already begun to sour while it was in the possession of the seller. Nevertheless, he is not held liable, because he can claim that it soured because of the containers that belonged to the purchaser (Bava Batra 98a).
Sefer Me’irat Einayim 230:10 states that this law applies even if the purchaser used new, clean containers. For it is possible the wine began to spoil when - and because - it was poured from the sealed containers of the seller.

In which instance, a large quantity of wine will not be used immediately. For this reason, the wine should be of a quality that can be stored without turning sour. Nevertheless, since the responsibility for the spoilage can be attributed to the containers, the seller is not held responsible.

And it is nullified. The Tur and the Ramah (Choshen Mishpat 230:5) state that this law applies only when the purchaser says that he needs the wine for cooking.

Or if he told him: “I will drink it a little at a time [Shulchan Aruch (Choshen Mishpat 230:6)].”

The purchaser is allowed to nullify the transaction, because the product that he received - wine that has soured - is not what he desired to purchase.

The Kessef Mishneh explains that the Rambam rules that if the wine goes sour within three days, the seller is held responsible. The Tur and the Ramah (loc. cit.) maintain that everything depends on the local custom. If the wine became sour within the time one would assume that it would be drunk, the seller is liable. if it became sour afterwards, he is absolved from responsibility. See the following halachah and notes.

Since initially the quality of the wine was acceptable, the seller is considered to have met his obligation, even though he knew that his wine might tum sour. He was not given any indication that the purchaser might set aside the wine for later use, and he therefore did not have to warn him (Maggid Mishneh).

I.e., the beer was sold together with its container. If, however, the beer is transferred to a barrel belonging to the purchaser, the seller is no longer liable (Maggid Mishneh; Sefer Me’irat Einayim 230:14).

The Tur (Choshen Mishpat 230) makes a distinction between wine and beer, stating that wine is more dependent on the fate of its owner. Therefore, after it remains in the purchaser’s possession for a day, it can be said that the purchaser’s fate caused it to sour. Beer, by contrast, is not as dependent on its owner’s fate. Hence, a three-day period is granted.
The Kessef Mishneh maintains that the Rambam does not make such a distinction. The Bayit Chadash (Ciwshen Mishpat 230) differs and maintains that the Rambam also follows the reasoning stated by the Tur.

For the chemical process had already begun before the sale. At the time of the sale, it had not become noticeable yet [Tur (loc. cit. 230)].

Therefore, he and not the seller must suffer the loss.

Since the purchaser informed the owner that he would not be selling the wine all at once, the seller should have given him wine that would keep for an extended time.

I.e., he makes a different hole to which to move the spout. This could cause the wine to begin to sour. The commentaries question why this factor is not mentioned in Halachah 3 as well.

And he may not return it if it sours.

The Rambam (based on Bava Batra 98a) is referring to a particular business convention. A person would purchase wine or other merchandise from a merchant who made both wholesale and retail sales, on the condition that he sell it in another location. He would pay the wholesaler after he returned. Thus, although he purchased the wine, since he purchased it with the intent of transporting it, the wholesaler would have to provide him with wine that would keep over the duration of this journey.

Since the wholesaler would protest if the purchaser sold it in his locale, for it would prevent him from making retail sales, he must accept responsibility that the wine retain its flavor until it reaches its destination.

Significantly, when quoting this law, the Shulchan Aruch (Choshen Mishpat 230:10) substitutes mevushal (“boiled”) for mevusam (“fragrant”).

After Shavuot, the summer heat reaches its peak and it is possible for even fine wine to lose its flavor and aroma. In Eretz Yisrael, the grape harvest takes place during the month of Tishrei, so the wine must retain its flavor for approximately eight months.
The Tur and the Ramah (Choshen Mishpat 230:10) state that this law applies only when the wine is kept in the seller’s barrels.

Since wine was usually made during Tishrei, the intent is that the wine must retain its flavor for a full three years (Maggid Mishneh).

Even when the local custom differs with the above guidelines, the local custom takes precedence. The rationale is that any stipulation made by the participants in a business transaction is binding. Since the custom is well-known, it is assumed that both parties agreed to it and considered it as part of their business transaction. See Chapter 18, Halachah 12.

The rationale for this ruling is that by saying “this,” the seller indicates that he is selling the wine as is, alluding to the fact that there is something amiss with the wine that he is selling. Hence, although the seller knows that the wine will be used for cooking and therefore must retain its flavor over time, the purchaser must be willing to accept that a certain amount is of lesser quality (Sefer Me’irat Einayim 230:1).

And may return it.

By stating that the wine is fit for cooking, the seller implies that it is of high quality and will retain its flavor over time.

Even though the seller did not say “for cooking,” since he is selling him only one barrel, that barrel must be of high quality (Maggid Mishneh).
Significantly, the standard texts of the Shulchan Aruch (Choshen Mishpat 230:2) write “barrels,” using the plural, but that appears to be a printing error.

From the Rambam’s wording here and in his Commentary on the Mishnah (Bava Batra 6:2), the intent appears to be wine of ordinary quality. The Tur and others interpret this to be referring to wine that has already begun to sour.

Since the seller did not mention that it was wine, nor that it would be used for cooking, the purchaser should have investigated the item he was purchasing before completing the transaction.
Kin’ at Eliyahu mentions that in all the transactions mentioned above, the laws of ona’ah apply. Thus, if the purchaser pays one sixth more than the value of the product purchased, that sixth should be returned to him, and if the unfair gain is more than a sixth, the transaction can be nullified.