Reference for Bava Metzia 144:11

ולמאן דסליק אדעתין מעיקרא דאמר ר' יוחנן אפי' כדורמוס הזה אלא מתני' דקתני אין פוסקין על הפירות עד שיצא השער יצא השער פוסקין היכי משכחת לה מתניתין בחיטי דאכלבי וארבי דמשוך תרעיה טפי

Now, on the original hypothesis that R. Johanan referred even to a great fair, how is our Mishnah conceivable, which teaches, A MAN MUST NOT FIX A PRICE FOR PRODUCE UNTIL THE MARKET PRICE IS KNOWN; ONCE THE MARKET PRICE IS ESTABLISHED, A FIXED PRICE MAY BE AGREED UPON? — Our Mishnah relates to wheat in granaries and ships, whose fixed price extends over a long period.<span class="x" onmousemove="('comment',' When the wheat has been stored, or sufficient has been imported, its price is stabilised and there is no fear of appreciation, which may result in an appearance of interest. ');">²¹</span> Our Rabbis taught: One may not contract for commodities until the market price is out; once the market price is established, a contract may be entered into, for even if one [the vendor] has no stock, another has. If the new supplies were at four [se'ahs per <i>sela'</i>] and the old at three, a contract may not be made until the price has been equalised for the new and old.<span class="x" onmousemove="('comment',' New supplies were cheaper, because they were not yet fully dried. Now the purchaser, though paying early, does not receive the wheat until that too becomes old, and if he contracts for the whole at the price of new, he receives interest. Therefore he must wait until the same market price is fixed for both. ');">²²</span>

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