Having already warned above (in Pasuk 15) against perversion of justice, why does the Pasuk repeat it here?
Rashi: The Torah here is issuing a warning to every individual who does business not to weigh and measure dishonestly, since he has the status of a judge, and a judge who perverts justice is described as 'hated, abominable, banned, and despicable;' he contaminates the land, desecrates Hashem, drives the Shechinah away, causes Yisrael to fall by the sword and exiles them from their land. 1
R. Bachye: Pasuk 18 is referring to the justice discussed in Parshas Mishpatim, the current Pasuk refers specifically to the laws that nations initiate independently in connection with weights and measures. 2
Seforno: With reference to 19:33:2:1: Having mentioned Ona?ah, which referred S?tam to Ona?as Devarim, the Torah now discusses the other side of Ona?ah (Ona?as Mamon) in detail. 3
Oznayim la'Torah: Consequently, one should make a point of shopping by a shopkeeper who is scrupuously honest - Kal va'Chomer from Beis-Din, in connection with which Chazal said 'Halach achar Beis-Din Yafeh' despite the fact that the majority of Dayanim do not take bribes. See Oznayim la'Torah.
R. Bachye: Similar to the Pasuk in Sh?mos 15:25 ?Sham Sam lo Chok u?Mishpat ? ?. Refer to Sh?mos 15:25:3:1.
See Peirush on the Seforno.
What is the definition of "Midah" of "Mishkal" and of "Mesurah"?
Rashi #1 (citing Bava Metzi'a, 61b), R. Bachye #1 and Rashbam: "Midah" refers to measuring land, 1 "Mishkal", to weighing food 2 and the like and "Mesurah", to measuring liquids. 3
Rashi #2 (in Bava Metzi?a 61b) and R. Bachye #2 (citing Bava Metzi'a 61b): "Mishkal" forbids placing one's weights in salt 4 . "Mesurah" forbids being ?Marti'ach.? Pouring the liquid quickly, causing it to foam and consequently, to shortfall the customer. 5
R. Bachye #3 and Moshav Zekenim #1 (both citing Bava Basra 89b): Mesurah is a tiny measure, one thirty-sixth of a Log (between nine and sixteen milliliters). 6 [And if the Torah is particular about a thirty-sixth of a Log, how much more so is it particular about half a Hin, a third of a Hin, a Log, half a Log and a third of a Log ? R. Bachye citing Bava Basra]).
Moshav Zekenim #2: Mesurah is a rod (used to level off the contents in a container). 7
Targum Yonasan: A warning against using the same measuring-rope when measuring for two partners, for one partner in the summer, and for the other, in the winter.
Bava Metzi'a, (Ibid.): A warning against dipping one's weights in salt - Refer to Answer #2, note 1.
Targum Onkelos and Targum Yonasan translate "Mesurah" as volume - with regard to the different customs of selling, heaped or level. See Bava Basra, 89a. Refer also to answer #2, note 2.
Riva: Rashi explains that this makes the weight heavier and the Torah is warning the purchaser. R. Bachye and R. Tam citing the Yerushalmi, explains that it makes the weight lighter, and the Torah is warning the seller.
The Shitah Mekubetzes however explains that it entails heating the liquid, which (temporarily) increases the volume. See also Torah Temimah, note 264.
One Log = 6 egg-volumes.
The Gemara in Bava Basra 89b rules that it should be made of the wood of an olive or nut-tree. R'mah: Because if it is made of metal, it sinks into the Kli, and the measure will not be full. Re'em, in Shitah Mekubetzes: The measure will remain heaped, and the seller loses.
Why does the Torah mention specifically ?Mesurah??
R. Bachye: With reference to 19:35:2:2 ? To render the sinner Chayav from the moment he pours the liquid, 1 even if he does not ultimately sell it ? just as the Chachamim said in Bava Metzi?a 61b. 2 3
R. Bachye: Otherwise, there would be no Chidush ? seeing as, assuming it is worth a P?rutah, it is obvious that he transgresses ?Lo Sigzol?.
See R. Chavel?s footnotes. See also R. Bachye who elaborates further.
R. Bachye (citing Bava Basra 85b): The Torah is more stringent concerning the Isur of Midos (measurements) than that of Arayos ? because, as opposed to Arayos, where, as long as a baby has not been born, one can do Teshuvah ? (by Midos) one cannot do Teshuvah. Refer to 20:2:1:2*. See also R. Chavel?s footnotes, note 97.
