Bava Metzia, Chapter 4, Mishnah 11. In this Mishnah, we return to the laws of commerce, and it deals with the prohibition of deceptive sales tactics.
"Ein me'arvin peiros bepeiros" - one may not mix produce with produce:
It is forbidden to mix two types of grain together. Two approaches are stated in explaining this prohibition:
The approach of Rashi and the Bartenura: This refers to someone who agreed with the buyer to sell him grain from field A, and he is not permitted to supply him with grain from field B - even if the two grains are completely identical to one another. Since he specified A, he is obligated to provide A. This is derived from their words and is brought as halachah.
The approach of the Rosh (and the Tiferes Yisrael takes this position as well, and it is also brought as halachah): This refers to a mixture involving inferior-quality grain. The buyer purchased type A grain, and the seller supplies him with a mixture of ninety-five percent type A and five percent type B in order to get rid of the inferior merchandise. This is forbidden.
"Af lo chadashim bechadashim" - nor even new with new: even when dealing with grain that is more or less equal in quality. According to Rashi, this refers to identical produce coming from two different fields; and according to the Rosh, it refers to two new batches of grain, except that one is of better quality than the other.
"Ve'ein tzarich lomar chadashim biyeshanim" - and needless to say, new with old: It is obvious that one may not add recently harvested grain to older grain. As grain ages, it dries out, and it is desirable for harvested grain to dry out for storage purposes. Once it dries, its volume decreases and it becomes denser, since the moisture within it has decreased. As a result, more flour can be produced from a kilogram of old grain, because it contains less water. One who adds new grain is therefore deceiving the buyer, since the final yield will be less than that of old grain.
"Be'emes bayayin hitiru le'arev kasheh berach" - in truth, with wine, they permitted mixing harsh with soft:
Wherever the Mishnah uses the term "be'emes" (in truth), its intention is an undisputed rule. Here, they permitted mixing, at the time of pressing the grapes before placing them into barrels for fermentation, a little bit of the harsh wine - the stronger wine - into the soft wine, since the harsh one is superior, and the fermentation and final product will turn out well. "Mipnei shehi mashbichaso" - because it improves it: The mixture is for the benefit of the wine, for the benefit of the product, and for the benefit of the buyer.
It is worth noting that this should not be done at a later stage, after each of the wines has fermented in its separate barrel. Mixing at this stage will create two different flavors mixed together, and its result is poor. However, at the time of pressing, when the wines ferment together, the flavors integrate and blend with one another, and the product is superior than it would be without any mixing at all. For this reason, a mixture of this kind is permitted.
"Ein me'arvin shimrei yayin bayayin" - one may not mix wine dregs into wine:
One may not mix the dregs - the sediment at the bottom of the barrel - from one barrel to a second barrel, because this adds new flavors and new bacteria, causing the second barrel to spoil. This is similar to someone who has leftover milk remaining in a first container: it is not poured into a fresh milk container, since two types of milk of different ages and with different bacteria are not mixed. The same applies to wine.
"Aval nosen lo es shemarav" - but he may give him its dregs:
First, we must understand the logic behind this. When the wine reaches the bottom of the barrel, a liquid remains that cannot be sold, as it is full of sediment. If all this were poured into a second vessel full of wine, one could extract more liquid from it - and this is the motive for mixing it. But this is forbidden, because the taste is ruined and the product becomes inferior.
What is permitted is this: Someone who has a barrel of wine with dregs that settled at the bottom may mix them and return them into that same barrel, since this does not harm the taste, and they will settle again after being poured into the buyer's vessel. There is no inferior product here, but rather standard practice.
"Mi shenisarav mayim beyeno, lo yimkarenu bachanus ela im ken hodi'o" - One whose wine became mixed with water may not sell it in a store unless he informs him:
A person who added water to his wine during the pressing stage diluted his wine and ended up with a weaker wine. This method is economical, and it was commonly done. He may not sell this diluted wine in a store unless he informs the buyer that it contains ten or fifteen percent water. In such a case, it is permitted, since he informed him and the buyer understands the nature of the purchase, and there are even those who prefer such wine.
"Velo letagar af al pi shehodi'o" - And not to a merchant even if he informs him:
One may not sell wine diluted with water to a merchant who will sell it further, even if he informed him of this, "she'eino ela leramos bo" - for it is only to deceive with it - we are concerned that he will use it to mislead his customers, who will think they are receiving pure wine while they are actually receiving wine diluted with water.
Why are we concerned about deception here?
After all, we have an established rule that we do not assume a person to be a deceiver, and we judge people favorably. Why, then, when the wholesaler sells to the retailer and informs him that the wine contains fifty percent water and that he must inform the customers, are we concerned that he will deceive?
The answer is that this assessment was stated by Chazal regarding human nature and psychology: The retailer never saw the pouring of the water and did not dilute it himself, and since it is out of sight, out of mind, it will be easy for him to find a justification for himself that there is no need to tell the customers, as he did not see it with his own eyes and cannot be sure that it actually happened. Therefore, he will come to deceive his customers, and it is forbidden to place the means to do so in his hands, because otherwise it would be at the very least in the category of assisting in a transgression.
"Makom shenahagu lehatil mayim beyayin, yatilu" - A place where they are accustomed to put water into wine, they may put it:
In a place where this is the accepted custom, it is permitted. This is true even today: Grape juice that is commonly sold is usually not pure grape juice but rather reconstituted juice, and ground beef is labeled as containing ten percent water. When everyone knows that the preparation method in that area is to add water during the stage of filling the barrels and storage, this is a normal and permitted situation, and there is no room for concern - since this is an accepted business practice and it is everyone's expectation.
In summary: The core point of the Mishnah is that it is forbidden to do things that mislead the customer. As long as the customer is not misled, the practice is permitted.