Halakhah su Esodo 13:7

That chamets not be found to us on Pesach: That chamets should not be seen in all of our dwellings all seven days of Pesach, as it is stated (Exodus 13:7), "and no chamets shall be seen with you, and no leaven shall be seen with you in all your territory (seven days)." And these are not two negative commandments about two topics, but rather one negative commandment; as they, may their memory be blessed, said (Beitzah 7b), "The verse opened with chametz and ended with leaven, to say to you that chamets is leaven" - meaning to say [that] there is no difference between chamets itself and something that causes it to become chamets.

A Jew who has chametz in his possession on Pesach, continually transgresses the law, "No chametz must be seen in your possession" (Ex. 13:7) and "No chametz may be found in your home" (Ex. 12:19). Benefit of such chametz is forever forbidden, even if he nullified it before Pesach.¹Many later Poskim rule that this law applies, even if you made the search for chametz and also nullified the chametz, but others are lenient and rule that if you both searched for and nullified the chametz, it is not forbidden for benefit, since you fulfilled all that was required of you. Therefore, in a situation where there is a potential for great losses, this lenient opinion may be relied on. (Mishnah Berurah 448:25; also Biyur Halachah) Therefore, if you own a great deal of chametz that you cannot do away with, you must sell it to a non-Jew before Pesach, while you are still permitted to derive benefit from it. You should not treat the matter of selling chametz as a routine formality. Rather, it should be your firm intention to actually sell the chametz to the non-Jew, in a firm and binding sale. You should not sell it for more than it is worth. After Pesach you demand from the non-Jew that he pay his debt, and when he replies that he has no money, you ask of him to resell the chametz to you (together with the room) for so-and-so-much. You should not make a mockery out of this matter, rather, it should be handled in a business-like manner.

That He prohibited us from having weights and measures that are deficient, with us in our homes - even though one does not buy and sell with them. And that is His, may He be exalted, saying, "You are not to have for yourself in your pouch varying weight-stones, large and small" (Deuteronomy 25:13); and likewise, "varying ephah measurements" (Deuteronomy 25:14). And the language of the Gemara, Batra (Bava Batra 89b), is, "It is prohibited for a person to keep in his house a measure that is deficient or inflated, even if [he only uses it as] a chamber pot for urine." And you should not think that His saying, "You are not to have for yourself [...] "varying ephah measurements"; and "You are not to have for yourself [...] varying weight-stones" - [signifies] that they are two [distinct] negative commandments. Indeed, it is coming to round out the laws of the commandment, so that the types of measures are explained - and they are weight and measurement. It is as if He would say, "You should not have two measures with you - not for measuring, and not for weighing," as we explained in the Positive Commandments (Sefer HaMitzvot, Positive Commandments 208). And His saying, "You are not to have for yourself [...] varying weight-stones"; and "You are not to have for yourself [...] varying ephah measurements" - is like His saying, "You shall not charge interest to your brother; interest of money, interest of food, interest of anything upon which interest can be charged" (Deuteronomy 23:20) - which is one negative commandment that includes many types, all of which have the very same content. And it is not from the repetition of language that there is an expansion of commandments, as we discussed earlier in Principle 9. And something like this negative commandment already came before us; and that is His saying, ""no chametz may be seen [...], and no leaven may be seen" (Exodus 13:7). (See Parashat Ki Tetzei; Mishneh Torah, Theft 7.)

A mixture including hametz, on its account one transgresses the prohibition on seeing hametz (Exodus 13:7) and the prohibition on finding hametz (Exodus 12:19), e.g., brine, bread/milk preserve, beer, and the like. But something that is a mixture containing hametz but is not fit for consumption, one is permitted to keep on Pesah, e.g., the leather-worker's mixture that one put into it skins and flour, even if one put them together an hour before the time of destruction of hametz, it is permitted to keep it. But if one did not put the skins in but did put the flour in: three days before the time of destruction of hametz, one is permitted to keep it since it is lost and spoiled; within three days, one must destroy it. Similarly, eye salve, bandage, rag, or medicine that one has added hametz to, it is permitted to keep them over Pesah since they have lost the form of hametz.

And Rambam wrote (Sefer Hamitzvot LaRambam, Mitzvot Lo Taase 272), "And do not think that they are two [distinct] commandments since they are two negative statements [in the Torah]. Indeed, [the two statements] have come to complete the laws of the commandment, such that two types of size are included - and they are the weight and the measure. [It is] as if it stated, 'You should not have two sizes, not in measurement and not in weight.' [It is] like 'You shall not take interest from loans to your brother, whether in money or food or anything else that can be taken as interest' (Deuteronomy 23:20) - which is all one negative commandment. As it is not by the duplication of expressions that [the number of] commandments increase, when it is all one matter. And so [too with] 'no leavened bread shall be found with you, and no leaven shall be found' (Exodus 13:7), which is [all] one negative commandment, since it is [all] one matter. Rather, [the two expressions] are stated to complete the elucidation of the matter."