Pesachim 40B

מנקטא ליה אימיה בארבי.

that his mother would bring him wheat in a trough. In other words, she would guard the wheat grains from when they were harvested by placing them in vessels and guarding them until Passover.

ההוא ארבא דחיטי דטבעא בחישתא, שריא רבא לזבוני לגוים.

The Talmud relates that there was a certain boat carrying wheat, which capsized before Passover in the Ḥishta River. Rava permitted its owners to sell the recovered grain to non-Jews before Passover.

איתיביה רבה בר ליואי לרבא: בגד שאבד בו כלאים ⁠— הרי זה לא ימכרנו לגוי, ולא יעשה בו מרדעת לחמור. אבל עושין אותו תכריכין למת.

Rabba bar Levai raised an objection to the opinion of Rava from a baraita: With regard to a garment in which diverse kinds, a prohibited mixture of wool and linen, has been lost, i.e., a wool garment into which a linen thread was sewn or vice versa, one may not sell it to a non-Jew; and one may not even fashion it into a saddlecloth for a donkey. It is prohibited to do so lest one remove a piece of this garment and sew it onto his own clothing. However, one may fashion it into a shroud for a dead body, as there is no concern that he will remove it from the dead.

לגוי מאי טעמא לא ⁠— לאו משום דהדר מזבין לישראל?

The Talmud clarifies this issue: What is the reason that one may not sell it to a non-Jew? Is it not due to the concern that the non-Jew will resell it to a Jew? Since the mixture of wool and linen is not visible, it is possible for a Jew to use this cloth unawares. The same concern applies to grain: It is not apparent that the grain that capsized in the river is prohibited. It should therefore be prohibited to sell this wheat to non-Jews, lest they resell it to Jews.