Is it valid for a traveler to pray Maghrib and Isha shortened and combined if he intended to travel and passed through the city to collect some belongings, then prayed Maghrib in congregation, and then prayed Isha shortened before leaving the city?

A traveler is not permitted to avail himself of the concessions of travel before he embarks on his journey and departs from the inhabited area of his town. Therefore, if someone shortens the prayer while still in his city, his prayer is invalid, and he must repeat it in full, as four rak'ahs.