Reference for Bava Kamma 205:4

רב כהנא יהב זוזא אכיתנא לסוף אייקר כיתנא זבניה מרוותיה דכיתנא אתא לקמיה דרב א"ל מה אעביד איזיל אישקיל זוזאי א"ל אי כי קא זבני אמרי האי כיתנא דכהנא הוא זיל שקול ואי לא לא תשקול

and R. Kahana had never pulled the flax [to acquire title to it],<span class="x" onmousemove="('comment',' In accordance with Kid. I, 5 and B.M. IV, 2. ');">¹¹</span> and Rab was following his own reasoning, for Rab [elsewhere] stated: Advance payment<span class="x" onmousemove="('comment',' I.e., when the vendors received the money from R. Kahana they were not yet in possession of flax at all, but acted in accordance with B.M. 72b. ');">¹⁰</span> [at present prices] may be made for [the future delivery of] products,<span class="x" onmousemove="('comment',' I.e., where the very products stipulated for are to be delivered. ');">¹²</span> but no advance payment [at present prices] may be made [if the value of the products will subsequently be paid] in actual money<span class="x" onmousemove="('comment',' As this case would amount to the handing over of a smaller sum of money to be paid by a bigger amount and would thus appear to act against the spirit of the prohibition of usury. ');">¹³</span>

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