Is it valid to pray the 'Isha prayer shortened and combined with the Maghrib prayer (Jam‘ Taqdim) when 70 km away from home while returning from a journey, and what is required to be done?

It is permissible for a traveler on a lawful journey to combine two prayers that share a time slot (Dhuhr and Asr, or Maghrib and Isha), either by performing them early (Jame' Taqdeem) or delaying them (Jame' Ta'kheer). Imam Ahmad narrated from Ibn Abbas that the Prophet, peace and blessings be upon him, used to combine Dhuhr and Asr when the sun had passed its zenith while he was in his dwelling, and he would combine Maghrib and Isha when Maghrib was due while he was in his dwelling. If it was not yet due, he would ride until Isha was due, then combine them. Based on this, the questioner's prayer is valid as long as he has not entered the buildings of the town where he resides.