Praise be to Allah.
It is obligatory to pay zakaat al-fitr before the Eid prayer, because it is proven in al-Saheehayn from the hadeeth of Ibn ‘Umar (may Allah be pleased with him) that the Prophet (blessings and peace of Allah be upon him) commanded that it should be given before the people went out to pray.
If a Muslim does not give it before the prayer, as is required of him, it is not waived in his case; rather he must pay it even if that is after the prayer.
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) was asked:
I did not pay zakaat al-fitr because Eid came suddenly, and after Eid al-Fitr I did not have any free time to ask about what I should do in this case. Is it waived from me or do I still have to pay it?
Zakaat al-fitr is obligatory. Ibn ‘Umar (may Allah be pleased with him) said: The Messenger of Allah (blessings and peace of Allah be upon him) enjoined zakaat al-fitr. It is obligatory for every single Muslim, male and female, young and old, free and slave. If it so happens that Eid comes suddenly before he is able to pay it, then he must pay it on the day of Eid, even if that is after the prayer, because if a person misses the time for an obligatory act of worship due to some excuse, then he must make it up when that excuse no longer applies, because the Prophet (blessings and peace of Allah be upon him) said: “Whoever forgets a prayer, let him offer it as soon as he remembers, for there is no expiation for it other than that.” Narrated by al-Bukhaari (597) and Muslim (683).
And he recited the verse in which Allah says (interpretation of the meaning): “and perform As‑Salaah (Iqaamat‑as‑Salaah) for My remembrance” [Ta-Ha 20:14].
Based on this, you have to pay it now. End quote.
Majmoo’ Fataawa Ibn ‘Uthaymeen, 20/271.