Is the prayer of a traveler valid if he intended to delay Maghrib so as to pray it shortened with a congregation praying 'Ishaa', and he joined them in the second rak'ah, knowing this during the prayer, which would make his prayer an even number of rak'ahs and not an odd number?

It is permissible for a traveler, if he finds a resident praying Isha, to pray behind him with the intention of Maghrib.

If the resident is at the beginning of his prayer, the traveler prays three rak'ahs with him, and then he has the choice between leaving the imam and saying the taslim or sitting and waiting for the imam to say the taslim with him.

If the resident has already prayed one rak'ah, the traveler prays and says the taslim with him, and it does not harm him that the end of his prayer is an even number [of rak'ahs].

If the resident has already prayed more than one rak'ah, the traveler prays what he catches up with him, then he completes what he missed to the extent of the Maghrib prayer after the imam's taslim.