Reference: Fataawa Ramadhaan v2 p932 no.915
Fataawa as-Siyaam libni Jibreen p187
Question: If one does discharge the Zakaatul-Fitr before the ‘Eed prayer, is he relieved [of this obligation]?
Response: One who does discharge the Zakaatul-Fitr before the ‘Eed prayer then he is a sinner and he is not relieved [of this obligation], rather he must discharge it afterwards.

