If someone does not complete their missed fasts from the previous ramadan, and they are not able to make them up before the next ramadan, does this person have to make up the fasts after ramadan AND pay the required amount, or will it suffice to make tawbah and only make up the fasts after the next ramadan?
The correct opinion is that if a person has fasts owing from another Ramadan and they are not able to complete these fasts before the next Ramadan, then they fast the second Ramadan in full. Then when the second Ramadan ends, they proceed to make up the fasts owing on them from the first Ramadan. They should repent to Allah for delaying the fasts until that point and resolve not to do that again. There is no need for them to pay a Kaffarah (expiation) according to the correct stance of the scholars.
This is the opinion of Ibraahim an-Nakha’i (d. 96 H.) from the Salaf, Imam al-Bukhari (d. 275 H.), Imam an-Nawawi (d. 676 H.) in al-Majmoo’, Imam ash-Shawkaani (d. 1250 H.) in as-Sayl al-Jaraar, Shaikh Muqbil (d. 1421 H.), and others (may Allah have mercy upon them all). This is also the view of Shaikh Yahya al-Hajuri [See: Ithaaf al-Kiraam bi Ajwibah Ahkaam az-Zakat wal-Hajj was-Siyaam (pg. 385-386)]
And Allah knows best
Faisal bin Abdul Qaadir bin Hassan