I stay in Kolkata as a muqim. When I was in Delhi as a Musafir, I board the flight after praying Maghrib and waqt for Isha started in Delhi itself when I was in the flight. I decided that I would pray Isha after reaching home (Kolkata) at around 10 PM. If I would pray Isha in Delhi itself, I would pray Qasr. Now I am back to home, do I need to pray Isha as Qasr or as a muqim (4 rakah) for that Isha which I did not pray in Delhi.
بسم الله الرحمن الرحيم
(Fatwa: 738/619/L=6/1439)
In case of offering qasr or full salah, the last time is taken into consideration, not the starting time. Hence if you reached home at a time when the time of Isha was remaining then you will have to offer full salah.
Allah knows Best!
Darul Ifta,
Darul Uloom Deoband