We used to pay the zakat of fitr during the time of the Messenger of Allah ﷺ as a saa of barley, a saa of dates, a saa of raisins, or a saa of dried curd. When Muawiya came to power and high-quality wheat from Sham became available, he held the view that one mudd of it was equivalent to two mudds of the other foods.
حدثنا عبد الرزاق أخبرنا سفيان عن زيد بن أسلم حدثنا عياض بن عبد الله بن سعد بن أبي سرح
كنا نؤدي صدقة الفطر على عهد رسول الله ﷺ صاعا من شعير صاعا من تمر صاعا من زبيب صاعا من أقط فلما جاء معاوية جاءت السمراء فرأى أن مدا يعدل مدين
Its isnad is sahih, and this is a sound understanding from Abu Saeed, and it conveys the practice of the companions who understood the religion from the Messenger of Allah ﷺ. This is what I always give as a fatwa. Whoever finds wheat priced like barley among them should pay more, according to his wealth, because all Muslims pay the same as one another.