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Tzemach Tzedek on Chametz that Accidentally Wasn't Sold (17 Nisan)

שו"ת צמח צדק שער המילואים ה:סח 17 Nisan 5602 (1842)

When a Jew forgets to sell chametz before Pesach, what to do?  Do we say that since he always sells his chametz he's included in the sale?