He—may Allah have mercy on him—was asked:

“Someone who is obliged to pay zakāt al-fitr knows that it is one sāʿ, but gives more, saying that the extra is voluntary charity. Is this disliked?”

He replied:

“All praise is due to Allah. Yes, this is permissible without being disliked according to most scholars, including al-Shāfiʿī, Ahmad and others. Only Mālik is reported to have considered it disliked. Giving less than the obligatory amount, however, is impermissible by scholarly agreement. As for whether the obligatory amount is one sāʿ, half a sāʿ, or more, there are two opinions. And Allah knows best.”