The reason for their divergence is the different ways they understood the ḥadīth of Abū Saʿīd al-Khudrī, who said:
“We used to pay Zakāt al-Fiṭr in the time of the Messenger of Allah (peace and blessings be upon him) with one sāʿ of food, or one sāʿ of barley, or one sāʿ of dried curd, or one sāʿ of dates.”
Whoever took the report to grant a free choice concluded: “Whichever of these he gives will discharge the obligation.” But whoever held that the variation was not a mere permission—instead reflecting either the payer’s staple food or the staple food most common in the land—adopted the second opinion.
As for the required quantity, the scholars agree that Zakāt al-Fiṭr may never be less than one sāʿ when it is paid in dates or barley, for that is firmly established in the ḥadīth of Ibn ʿUmar. They differed, however, over the amount due when the payment is made in wheat. Mālik and al-Shāfiʿī ruled that less than one sāʿ is insufficient, whereas Abū Ḥanīfah and his colleagues held that half a sāʿ of wheat suffices.
The root of their disagreement lies in the conflicting transmitted reports. One version from Abū Saʿīd al-Khudrī states:
“We used to give Zakāt al-Fiṭr in the time of the Messenger of Allah (peace and blessings be upon him): one sāʿ of food, or one sāʿ of barley, or one sāʿ of dried curd, or one sāʿ of dates, or one sāʿ of raisins.”
Its outward sense is that by “food” he meant wheat. Al-Zuhrī also narrated, from Abū Saʿīd through his father, that the Messenger of Allah (peace and blessings be upon him) said:
“For Zakāt al-Fiṭr: one sāʿ of wheat shared between two people, or one sāʿ of barley, or of dates, for each person.” Abū Dāwūd recorded this.
Ibn al-Musayyab likewise reported:
“During the lifetime of the Messenger of Allah (peace and blessings be upon him) Zakāt al-Fiṭr was half a sāʿ of wheat, or one sāʿ of barley, or one sāʿ of dates.”
Those who relied on these latter reports held that half a sāʿ of wheat is sufficient. Those who adhered to the outward purport of Abū Saʿīd’s ḥadīth and analogized wheat to barley required a full sāʿ of either.