By clear texts of the Qur’an and Sunna, and consensus of the scholars, it would be obligatory to make up those missed fasts–gradually, without undergoing hardship.
In the Hanafi school, a nursing woman unable to fast is obligated to make up the missed days, but does not have to give any type of expiation (fidya). [Ibn Abidin/Haskafi, Radd al-Muhtar `ala al-Durr al-Mukhtar].
In general, no expiation is needed for delaying making up one’s unperformed fasts until the next Ramadan comes in.
This is because it is not obligatory to make up one’s fasts immediately. [Halabi, Multaqa al-Abhur; Marghinani, al-Hidaya]. It is, however, recommended to make up one’s fasts as soon as possible, in order to clear one’s debt to Allah. [Shaykh Zada, Majma` al-Anhur Sharh Multaqa al-Abhur, 1.250; Ibn al-Humam, Fath al-Qadir, 2.354-355]
The reason it is not obligatory to make up one’s fasts immediately is because Allah’s command, namely His saying, “…and whosoever of you is sick or on a journey, (then fast the same) number of other days” [Qur’an, 2.185], is unconditioned, and the purport of unconditioned texts is decisive until another decisive text conditions it. [Mahbubi, al-Tawdih; Marghinani/Ibn al-Humam, Fath al-Qadir Sharh al-Hidaya, 2.354-355]
Thus, all we have been commanded to do is to fast any same “number of other days” to clear our debt.
However, one should be careful because if one dies without having taking reasonable means to make up the fasts–such that one’s delay was unexcusable–then one would be considered sinful. [Ibn Abidin, Radd al-Muhtar] One should keep track of unmade-up fasts and make arrangements for expiation payments to be given from the discretionary third of one’s wealth upon death, in case one dies unexpectedly, as death is wont to happen.
