Is it correct to give Zakat al-Fitr by taking four double-handfuls of food from the seller, in order to align with the Sunnah, even though a Sa’ is a unit of volume and not weight, and the weights of foods differ?

What you did concerning Zakat al-Fitr is sufficient if the aforementioned seller was of average hand size, for he estimated the sa' and found it to be four average handfuls, and Zakat al-Fitr is given from the staple food of the people during Ramadan.