Explanation: If he was Makneh to him a Se'ah of wheat, he may not say to him "I was Makneh to you a [Se'ah] Machuk (level)";

We are not Godesh (to give a heaping measure) in a place where they are Mochek. Even if the buyer is Muchzak, he cannot say "I will take a heaping measure." (Tosfos connotes that if the buyer and seller agree, they can do as they please, unlike the Rashbam.)

Explanation (Rivam): This was taught regarding Yados. If the string became Tamei, the scale (crossbar) became Tamei;

However, the pans (in which one puts the items weighed), which have a receptacle and the crossbar, they are Tamei (Mekabel Tum'ah) even by themselves.

The Gemara asks "what is the Chidush?", and answers that it must teach about its crossbar and ropes, which the Mishnah did not teach;

Even if the string was made properly, it is not considered a Yad unless the crossbar and ropes are made properly.

Explanation (R. Tam): This is only for weights used for weighing meat and oil, for the moisture sticks to them and dries up. It is not due to chips coming off, like Rashi explained.

Distinction: However, weights used for weighing silver and gold, one may make them of metal. However, one must cover them with leather.

Source #1 - Citation (Shabbos 79a): There are three leathers - Matzah (hide that was not salted or treated with flour or gallnuts), Cheifah (it was only salted) and Diftara (it was salted and treated with flour, but not with gallnuts);

It explains that [the Shi'ur to be liable for Hotza'ah of Matzah is] enough to wrap a small weight. If the weights are of stone or glass, why is leather needed?! Rather, they are of metal.

The reason is, because they are smooth, and [oil] does not stick to them, one can rinse them or clean them;

Source #2: A Mishnah (Bava Metzi'a 52a) says that a Sela (coin) that was disqualified, they enacted to weigh with it. This shows that one may make a weight from a metal coin! However, one must cover them with leather.

Limitation: Only a small weight used for weighing silver and gold, one must cover it with leather, for through a small chipping, a big loss results;

However, other big weights, used to weigh copper, iron and lead, there is not a big loss unless they chip too much, so one need not cover them.

Support: It connotes in Sanhedrin (68a) that they used to cover only the small weights, for it asks "a ball of wool, a mold (for making shoes), an amulet, or a holder for a pearl or a small weight, what is the law (do they can receive Tum'ah)?

Explanation #1 (Rashbam): This makes them heavier.

Objection (R. Tam): It connotes that we discuss a seller, similar to "Meshurah", that he may not cause the liquid to foam [for then he does not fill the measure with liquid]!

Explanation #2 (R. Tam): It is because salt makes the measure lighter, and absorbs [from it], like it says in the Yerushalmi that salt absorbs [from the] weights.

Question: Obviously, [this is forbidden]. This is absolute theft. We know it from "Lo Sigzol"!

Answer: The verse comes to make one liable from when he is Marti'ach, even though he did not measure yet.

Note: The Rashbam and most Meforshim explain that "Yarti'ach" is pouring a liquid quickly into a measure to make it foam, so the measure will appear full. This is done while measuring! It seems that Tosfos holds like the Aruch (Resech) brings from R. Chananel (also cited in Rashash Bava Metzi'a 61b), that it is heating the liquid before pouring it, to cause its volume to expand.

Support: We say like this in Bava Metzi'a (61b). "Why do we need a Lav for [fraudulent] weights to forbid covering them in salt? This is proper theft! It is to make one liable from when he does so [even before measuring with them]."