Is it permissible for a traveler to delay the Maghrib prayer and combine it with the 'Isha' prayer, by delaying both (jam' ta'khir), if the time for Maghrib comes before he reaches his destination while he is on his way back?

It is permissible for a traveler to combine Maghrib and 'Isha prayers by delaying them (Jam' Ta'kheer), as long as they intended to combine them before the time of the first prayer expired, and the journey continued until the time of the second prayer began. This is the view of the Hanbalis. If you combine the two prayers before reaching your place of residence, you shorten 'Isha. However, if you combine them after settling in your home country, you pray 'Isha in full, as the reason for shortening it no longer exists.