כִּ֣י תִקְצֹר֩ קְצִֽירְךָ֨ בְשָׂדֶ֜ךָ וְשָֽׁכַחְתָּ֧ עֹ֣מֶר בַּשָּׂדֶ֗ה לֹ֤א תָשׁוּב֙ לְקַחְתּ֔וֹ לַגֵּ֛ר לַיָּת֥וֹם וְלָאַלְמָנָ֖ה יִהְיֶ֑ה לְמַ֤עַן יְבָרֶכְךָ֙ יְהוָ֣ה אֱלֹהֶ֔יךָ בְּכֹ֖ל מַעֲשֵׂ֥ה יָדֶֽיךָ׃
네가 밭에서 곡식을 벨 때에 그 한 뭇을 밭에 잊어버렸거든 다시 가서 취하지 말고 객과 고아와 과부를 위하여 버려두라 그리하면 네 하나님 여호와께서 네 손으로 하는 범사에 복을 내리시리라
Rashi on Deuteronomy
ושכחת עמר [WHEN THOU REAPEST THY HARVEST IN THY FIELD] AND HAST FORGOT A SHEAF … [THOU SHALT NOT RETURN TO TAKE IT] — a sheaf, but not a stack of corn (Sifrei Devarim 283:1). Hence they (the Rabbis) said, “A sheaf that contains two Seahs of grain which one has forgotten in the field, does not come under the term of שכחה (a forgotten sheaf)” (Mishnah Peah 6:6).
Rabbeinu Bahya
לא תשוב לקחתו...למען יברכך ה' אלו-היך, “you must not turn back in order to take it;.....in order that the Lord your G’d will bless you.” G’d assigned a blessing for people who have the right thoughts. Sifri 283 adds that even if the handout, i.e. forgotten sheaf, became the property of the poor without deliberate intent by the farmer who had forgotten it, he will be blessed. How much more will he be blessed when he had arranged for the poor to become the recipient of this sheaf (or coin, etc.)?
Siftei Chakhamim
But not a heap. A heap is when it is the size of two seah.