In the lifetime of the Prophet ﷺ we used to give one Sa' of food or one Sa' of dates or one Sa' of barley or one Sa' of Raisins (dried grapes) as Sadaqat-ul-Fitr. And when Muawiya became the Caliph and the wheat was (available in abundance) he said, "I think (observe) that one Mudd (of wheat) equals two Mudds (of any of the above mentioned things).  

عَنْ أَبِي سَعِيدٍ الْخُدْرِيِّ ؓ قَالَ كُنَّا نُعْطِيهَا فِي زَمَانِ النَّبِيِّ ﷺ صَاعًا مِنْ طَعَامٍ أَوْ صَاعًا مِنْ تَمْرٍ أَوْ صَاعًا مِنْ شَعِيرٍ أَوْ صَاعًا مِنْ زَبِيبٍ فَلَمَّا جَاءَ مُعَاوِيَةُ وَجَاءَتِ السَّمْرَاءُ قَالَ أُرَى مُدًّا مِنْ هَذَا يَعْدِلُ مُدَّيْنِ