Is the prayer of one who intended to shorten it permissible behind an Imam who completed his prayer if he joined him in the first Tashahhud? And does the ruling differ if the Imam was in the last Tashahhud?

If a traveler prays behind a resident, the traveler must complete the prayer (i.e., not shorten it), even if they join the resident in the last tashahhud. If the traveler prays shortened behind the resident, their prayer is invalid and must be repeated.