Does the concession for skipping a Ramadan fast and making it up later apply only to people who are actually in the act of travelling or does it also apply to someone who is temporarily not at his residence (a musafir, the state of a person in which the rules for shortening salah apply)?
The scholars have differed on this. The majority of them draw analogy from the limit of curtailment of the prayer. A group of scholars view that "any" journey would qualify for this concession and the person may cease his fast. This view is attributed to Zahirites.
They differed over apparent meaning of the text where Allah says:
and whoever is ill or on a journey, [let him fast the same] then an equal number of other days. [Surah Al-Baqarah 2:185]
What is understandable though is that the permission of not fasting during the travel is for relief from hardships in the travel. However, not all journeys have hardships that entails this concession. Moreover, the companions agreed upon a limit for this i.e. the analogy from limit of curtailment of prayer. The minimum distance is that of a journey on a camel for two days which is approximately 80 km. This is what is the opinion of majority of the scholars.
Source: Minimum distance
Bidayat al-Mujtahid wa Nihayat al-Muqtasid, The Book on Siyam
It is only permissible to break fast when you are actually traveling
Fasting for] a limited number of days. So whoever among you is ill or on a journey [during them] - then an equal number of days [are to be made up]. And upon those who are able [to fast, but with hardship] - a ransom [as substitute] of feeding a poor person [each day]. And whoever volunteers excess - it is better for him. But to fast is best for you, if you only knew. 2:184
the specific translation of "3ala safarin" is while traveling. Allah also incourages one to fast even if they are travelling although it is permissible to break the fast he says if you are able to fast then do so. And lastely he says if only you knew fasing is much better for you. So if Allah is encourging us to fast even while we are travelling he would surley want us to be fasting when we are simply in a foreign place.