Is it permissible to give more than one Sa' of Zakat al-Fitr with the intention that the excess is charity?

It is permissible to give out Zakat al-Fitr with a Sa' (a measure) and an increase, and the increase is considered a supererogatory charity for which the giver is rewarded. There is no dislike (Karaha) in this according to the majority of scholars, such as Al-Shafi'i and Ahmad, while dislike was reported from Malik. As for giving less than the obligatory amount, it is not permissible by the consensus of scholars.