Just balances, just weights,.... Which were for such sort of things as were bought and sold by weight, and these were to be according to the custom and usage which universally obtained among them, or were fixed and settled by them; they were to be neither lighter nor heavier; they were not to have one sort to buy with, and another to sell with, which was not just, and was an abomination to the Lord, Pr 11:1; for "weights", it is in the original text "stones", for those were formerly used in weighing, and were with us: hence it is still in use to say, so much by the stone.
And according to Maimonides , the Jews were not to make their weights neither of iron, nor of lead, nor of the rest of metals, lest they should rust and become light, but of polished rock, and the like;a just ephah, and a just hin, shall ye have; the first of these was the measure of things dry, as corn, and the like, the latter of things liquid, as oil and wine; the one held three seahs or pecks, or ten omers, Ex 16:36; or, according to a nicer calculation, the ephah held seven gallons, two quarts, and half a pint; and the other, according to some, held three quarts; but, as more exactly calculated, it held a wine gallon, and a little more than a quart, See Gill on "Ex 30:24".
Some Jewish writers refer this to words, promises, and compacts, expressed by yea and nay, which they were to abide by; that their yea should be yea, and their nay, nay, Mt 5:37; that their affirmation should be just, and so their negation:I [am] the Lord your God, which brought you out of the land of Egypt; and therefore were under great obligations to observe his commands, as follows.
Hilchot Genibah, c. 8. sect. 4. Torath Cohanim apud Yalkut in loc. Maimon. & Bartenora in Misn. Sheviith, c. 10. sect.