Fiqh-us-Sunnah Volume 3, Zakaat and Fasting, Fiqh 3.089.

Most scholars believe that it is permissible to pay zakat ul-fitr a day or two before ‘id. Ibn ‘Umar reports that the Messenger, upon whom be peace, ordered them to pay zakat ul-fitr before the people went out to perform the ‘id prayers. Nafi’ reports that ‘Umar used to pay it a day or two before the end of Ramadan. However, scholars hold different opinions when a longer time period is involved. According to Abu Hanifah, it is permissible to pay it even before Ramadan. Ash-Shaf’i holds that it is permissible to do so at the beginning of Ramadan. Malik and Ahmad (in his well-known view) maintain that it is permissible to pay it only one or two days in advance.

The founders of the four accepted Islamic legal schools agree that zakat ul-fitr is not nullified simply by not paying it on its due date. If such is the case, it becomes a debt on the one responsible for it until it is paid. They also agree that it is not permissible to delay it until the day of ‘id, but Ibn Sirin and an-Nakha’i say that this can be done. Ahmad says: “I hope that there is no harm [in the delay of its payment].” Ibn Raslan says that there is a consensus that its payment cannot be delayed just because it is a type of zakah. Thus, any delay is a sin and is analogous to delaying one’s prayers without an acceptable excuse. This is proved by the following hadith: “If one pays zakat ul-fitr before the salah, it is considered an accepted zakah. If he pays it after the salah, it is considered an ordinary sadaqah.”