Zakat al-fitr doesn’t vary from one year to the other, as the Prophet (peace and blessings be upon him) stated that it is one saa` (about 2.176 kg of raw wheat). Thus, it was originally in kind. However, some Muslim jurists such as Imam Abu Hanifah and Ibn Taymiyah state that it is permissible to pay it in cash or in kind, according to the way that is better for the poor.
Sheikh Yusuf Al-Qaradawi states the following: Zakat al-fitr does not vary, as it is estimated by the Shari`ah and the Prophet (peace and blessings be upon him) as one saa` (about 2.176 kg of raw wheat). The wisdom behind estimating it by the saa`, in my understanding, is due to two reasons:
First, money was a rare means of dealing among the Arabs, especially the Bedouins. If you said to one of them “you should pay such-and-such sum of dirhams or dinars,” he would not have such amount of money. He would have only the common food such as dates, raisins, barley. This was what made the Prophet (peace and blessings be upon him) estimate it by one saa`.
Second, the purchasing power of money varies from one time to the other. Sometimes the value of the riyal, for example, decreases, and therefore its purchasing power decreases too. And sometimes the purchasing power rises in the market. Thus, estimating the amount of zakat al-fitr by a fixed amount of money would make it fluctuate and be unstable. That is why the Prophet (peace and blessings be upon him) fixed it by an amount that does not vary or fluctuate. This amount is the saa`, which is usually considered as a sufficient amount of food for one family for a day.
The Prophet (peace and blessings be upon him) identified the common foodstuffs in his age, which included (but was not limited to) dates and barley. Thus, scholars maintained that paying zakat al-fitr from the staple food in a certain area is permissible, whether this food is wheat, rice, corn, etc.
According to Imam Abu Hanifah, it is allowed to pay zakat al-fitr in cash. If the one who gives zakat al-fitr is rich, it would be better for him to pay more than the amount of one saa`, as the food is not limited to rice, but there should be meat, sauce, vegetables, fruit, etc. along with the rice.