The Torah in Parashat Kedoshim (19: 9-10) presents the obligation of “Leket” – leaving for the poor harvested stalks of grain which fell to the ground during the harvest – as both an affirmative command and a prohibition. It commands the farmer to leave these stalks for the poor, and forbids the farmer from taking them for himself. The Sefer Ha’hinuch lists these as two of the Torah’s 613 commands – the obligation to leave “Leket” for the poor (Misva 218), and the prohibition against taking the “Leket” for oneself (Misva 219).
As such, the prohibition against keeping “Leket” for oneself falls under the category of “Lav Ha’nitak La’aseh” – violations which can be rectified after the fact. If one transgresses this prohibition by taking the “Leket,” he has the opportunity to reverse his violation, and thereby avoid Malkut, by giving it to a needy person. As long as the stalks which had been taken in violation of this command are still extant, the field’s owner can avoid Malkut by giving them to the poor, which fulfills the affirmative command of “Leket” and rectifies the violation of the prohibition against taking “Leket” from one’s field.