A traveller joined two prayers together at the time of the earlier one, then he went back to his city before the time for the later prayer began

The concession regarding prayer that are connected to travel
is that one is allowed to join Zuhr and ‘Asr, and Maghrib and ‘Isha’, at the
time of the earlier or later of the two. 

The Prophet (peace and blessings of Allaah be upon him)
did not make it a habit to join his prayers on all his journeys; rather he
joined prayers sometimes and did not join prayers sometimes, praying each
prayer at its own time instead. 

Hence the scholars said: it is better for the traveller not
to join his prayers unless he needs to because it will be too difficult for
him to do each prayer at its usual time, even though the concession allowing
joining of prayers is established for every traveller. 

See al-Mughni, 3/131; al-Sharh al-Mumti’,
4/550-553 

Once it is established
that you come under the definition of a traveller, then it becomes
permissible to join the prayers, even if you know that you will return to
your city before the end of the time for the second prayer, or before the
time for it begins, because the evidence indicates that it is permissible
for the traveller to join his prayers; so as long as a person is travelling,
he may join his prayers.  

In al-Majmoo’ (4/180), al-Nawawi (may Allaah have
mercy on him) mentioned the difference of the scholars; there are two views
concerning the issue of a traveller who joins two prayers at the time of the
earlier one, but then reaches his destination and is no longer a traveller.
Does his joining of the prayers then become invalid, requiring him to repeat
the second prayer at its proper time, or not?

 Then al-Nawawi (may Allaah have mercy on him) said: The more
correct of the two scholarly views is that his joining of the prayers does
not become invalid, the same as if he were to shorten the prayers and then
reach his destination and no longer come under the heading of a traveller. 

Al-Muwaffaq Ibn Qudaamah (may Allaah have mercy on him) said
in al-Mughni (3/140): 

If a person completed two prayers at the time of the earlier
one, then the excuse for doing so ceased to exist after he had finished
praying, before the time for the later prayer began, that is still
sufficient and he does not have to repeat the later prayer when the time for
it comes, because his prayer was valid and he discharged his duty to perform
it, and that duty no longer applies after that. He discharged his duty at a
time when he had an excuse, and it is not invalidated by the fact that the
excuse ceases to exist after that. This is like the case of the one who does
tayammum, if he finds water after finishing his prayer (he does not have to
repeat it). 

Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) was
asked about a traveller who joined Zuhr and ‘Asr at the time of the earlier
prayer, although he knew that he would reach his destination before the time
of ‘Asr. Is this permissible? 

He replied: 

Yes, this is permissible, but if he knows or he thinks it
most likely that he will arrive before the time for ‘Asr, then it is better
for him not to join the prayers, because there is no need to do so. 

Majmoo’ Fataawa Ibn ‘Uthaymeen,
15/422. 

The Standing Committee for Issuing Fatwas was asked about the
same matter, and they replied: 

… If you joined Maghrib
and ‘Isha’ whilst travelling and shortened them, there is nothing wrong with
that, even if you reached your destination at the time of ‘Isha’. 

Fataawa al-Lajnah al-Daa’imah,
8/152. 

And Allaah knows best.