Eight seifim. Siman 162 forbade lending a measure against a measure: the price moves, and the lender may receive more than he gave. Here the same transaction is permitted — because one external fact changes it entirely: the commodity has, that day, a rate. The siman defines what a rate is, what invalidates it, what replaces it, and the condition always forgotten: the buyer must gain nothing beyond the goods. (Shulchan Arukh, Yoreh De’ah 175 — 8 seifim)
אין פוסקין על הפירות על שער של עיירות מפני שאינו קבוע אבל פוסקין על שער שבמדינה שהוא קבוע
One does not fix a price on produce by the rate of the towns, because it is not fixed; but one fixes by the rate of the great city, which is fixed. The whole siman is here: it is not the price that is in question, but its stability.
Shulchan Arukh, Yoreh De'ah 175:1
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Hebrew text of the eight seifim with a fluent English translation. The rate of the great city, new and old, the gleaners, the seller’s possession and the three operations, the bar on any gain, substitution, the more favourable rate and a sale made in prohibition — with the sugya of Bava Metzia 72b, the Tur, the Beit Yosef, the Shakh, the Taz, the Ba’er Hetev, the Pit’hei Teshuva and the whole Rambam.
In-depth pilpul: the basis of the heter between an excuse and an acquisition; Rashi against Tosafot on the towns’ rate and the Rama’s rule of method; the Rambam against the Tur on new and old, and the stringent ruling; why the gleaners have their own rate; the gemara’s two answers and the nafka mina the Taz draws from them; the quantity required of the seller; Rabbenu Tam against the Rashbam; and Tosafot’s answer on קרוב לשכר.
A master table of the eight seifim clause by clause, the axiom of the siman, the three reflex questions, the table of what makes a rate, the seller’s possession, the bar on any gain, substitution, the more favourable rate and the agent, the vocabulary, the golden rules, the mnemonic and the classic pitfalls.
The practical halakha according to the ש״ך, the ט״ז, the באר היטב and the פתחי תשובה, then contemporary cases. Note: the Admour HaZaken did not write a Shulhan Arukh on Yoreh De’ah — a level of psak, not “Daat HaRav”. The נקודות הכסף do not comment on this siman.