The Messenger of Allah ﷺ prescribed the sadaqah (alms) relating to the breaking of the fast as a purification of the fasting from empty and obscene talk and as food for the poor. If anyone pays it before the prayer (of 'Id), it will be accepted as zakat. If anyone pays it after the prayer, that will be a sadaqah like other sadaqahs (alms). (Using translation from Abū Dāʾūd 1609)
«فَرَضَ رَسُولُ اللَّهِ ﷺ زَكَاةَ الْفِطْرِ طُهْرَةً لِلصِّيَامِ مِنَ اللَّغْوِ وَالرَّفَثِ وَطُعْمَةً لِلْمَسَاكِينِ مَنْ أَدَّاهَا قَبْلَ الصَّلَاةِ فَهِيَ زَكَاةٌ مَقْبُولَةٌ وَمَنْ أَدَّاهَا بَعْدَ الصَّلَاةِ فَهِيَ صَدَقَةٌ مِنَ الصَّدَقَاتِ»