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?
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.