How is the debt of missed zakat al-fitr paid?

As with all acts of worship, the obligation of zakat al-fitr should not be delayed and must be fulfilled on time. However, if it was not paid at the proper time, it should be paid at the earliest possible opportunity.

According to Imam al-Shafi'i, Ahmad ibn Hanbal, and in one narration Imam Malik, the obligation of zakat al-fitr becomes due at sunset on the last day of Ramadan. According to Imam Abu Hanifa and some other mujtahid imams, it becomes due at dawn on the day of Eid. Nevertheless, zakat al-fitr may also be given during the month of Ramadan. In fact, it is preferable to give it before Eid so that the poor can meet their needs for the holiday. However, if zakat al-fitr has not been given by the morning of Eid, it must be paid during the days of Eid. If it is not paid on time and is delayed, it should be paid at the first possible opportunity. (Ibn Abidin, Radd al-Muhtar, 2/367)

In the Shafi'i school, it is permissible to give zakat al-fitr in the early days of Ramadan; however, unless there is a legitimate excuse, delaying it until after sunset on the first day of Eid is forbidden. (Nawawi, al-Majmu‘, 6/128)

According to the majority of jurists, delaying the payment of zakat al-fitr until after Eid is disliked (makruh), but the payment is still considered as fulfillment (ada) rather than as a make-up (qada). Some jurists, however, consider it forbidden to delay zakat al-fitr until after Eid and regard the payment as a make-up (qada). (Jaziri, al-Madhahib al-Arba‘a, 1/568)

Source: T.C. Diyanet İşleri Başkanlığı, Din İşleri Yüksek Kurulu