Yes, there is nothing wrong with that; the saa’ will be the obligatory zakaat al-fitr and anything more than that will be voluntary, and the giver will be rewarded for it.
Shaykh al-Islam Ibn Taymiyah (may Allaah have mercy on him) was asked about a person who owed zakaat al-fitr and knew that it was a saa’, but he gave more than that, and said that it was naafil. Is that makrooh?
Praise be to Allaah. Yes, it is permissible and is not makrooh according to most scholars, such as al-Shaafa’i, Ahmad and others. It was only narrated from Maalik that it is makrooh.
As for giving less than what is required, that is not permissible, according to scholarly consensus. End quote.
Majmoo’ Fataawa Ibn Taymiyah (25/70).