If a traveller decides to return part way through his journey, can he still shorten his prayers on his way back?

If a person decides to travel to some city, then on the way
he decides to go back to his own city, then one of two scenarios must apply:

1.

The distance from the place where he decided to go back to
his own city is equal to or greater than the distance at which it becomes
permissible to shorten prayers. In this case, he may avail himself of the
concession of travel on his way back, until he enters his city.

2.

The distance from the place where he decided to go back to
his own city is not equal to the distance at which it becomes permissible to
shorten prayers. In this case, he cannot avail himself of the concession of
travel on his way back, because on his way back he is regarded as having
started a new journey, so he cannot shorten his prayers unless the distance
is equivalent to the distance at which it becomes permissible to shorten
prayers, which is eighty kilometres.

This is the view of the madhhabs of the four imams.

Burhaan ad-Deen al-Bukhaari al-Hanafi said: If the traveller
sets out from his city, then decides to go back to his city for some reason,
and that happens before he has travelled the distance of three days, then he
should pray in the manner of non-travellers in the place where he is and
when travelling back to his city, because he has cancelled out the intention
of travelling, by deciding to return to his city before the rulings on
travel came into effect. Therefore he is no longer regarded as a traveller (musaafir)
and between him and his destination there is a distance of less than three
days. So he should pray in the manner of non-travellers on his way back.

But if he had travelled the distance of three days (or more),
then he decided to go back to his city, he may pray in the manner of
travellers, because the ruling on praying when travelling has already come
into effect by virtue of his completing that time, and the ruling remains in
effect until he is no longer travelling, when he reaches his destination

End quote from al-Muheet al-Burhaani fi’l-Fiqh an-Nu‘maani
(2/35) 

In at-Taaj wa’l-Ikleel (2/498) it says: Whoever sets
out on a journey in which it is permissible to shorten prayers, but he
travels a distance that does not make it permissible to shorten prayers,
then he goes back to his house for some reason, should offer his prayers in
full on the way back. End quote.

Al-Qarraafi said: If he travels for three parasangs (leagues)
then returns, he should offer his prayers in full on his return journey,
because it is a second trip that is shorter than the distance at which it
becomes permissible to shorten prayers.

End quote from adh-Dhakheerah (2/364)

[three parasangs or leagues = approximately 15 km]

Imam ash-Shaafa‘i said: If a man sets out from Makkah,
heading for Madinah, he may shorten his prayers, but if he becomes afraid on
the way, when he is in ‘Usfaan, and he wants to stay there, or to go out to
somewhere other than Madinah to stay there, if he gives up his previous
intention to travel to Madinah, I regard him as starting a new journey from
‘Usfaan.

So if the journey that he wants to take from ‘Usfaan is of a
distance shorter than that at which it becomes permissible to shorten
prayers, then he should not shorten his prayers, but if it is of a distance
at which it becomes permissible to shorten prayers, then he may shorten his
prayers.

Similarly, if he comes back from there, heading for Makkah or
some other city, I regard him as starting a new journey from that point. So
if where he is headed to is at a distance at which it becomes permissible to
shorten prayers, then he may shorten his prayers, but if it is not of a
distance at which it becomes permissible to shorten prayers, then he should
not shorten his prayers.

End quote from al-Umm (1/216).

An-Nawawi (may Allah have mercy on him) said: If he sets out
to a distant land, then on the road he decides to come back, he is no longer
travelling and it is not permissible for him to shorten his prayers so long
as he is in that place. But when he leaves that place, he has started a new
journey, and he can only shorten his prayers if he is setting out on a
journey of two “stages” [a unit of distance], whether he is going back to
his city, setting out for his original destination, or heading somewhere
else. This was stated by ash-Shaafa‘i in al-Umm, and our companions
are agreed on it.

Al-Majmoo ‘ Sharh al-Muhadhdhab (4/333)

[A “stage” = approx. 40 km]

Ibn Qudaamah said: If he sets out intending to travel a far
distance and he shortens his prayers, then he decides to come back, the
prayers he has already offered are acceptable and valid, but he should not
shorten his prayers on his way back, unless the distance he will travel on
his return journey is such that it makes it permissible to shorten the
prayers.

This was stated by Ahmad.

End quote from al-Mughni (3/110)