What if a person on a journey broke the fast of Ramadan?

This: https://islam.stackexchange.com/questions/48208/traveling-during-ramadan-should-i-still-fast made me think about the following question: What if a person -like somebody who is on a journey during Ramadan- who had the choice not to fast, but decided to fast before the end of the day broke his/her fast. This person started the day fasting in Ramadan (even if he/she had the permission to eat), but later maybe due to heat or whatever reason decided to break his/her fast before the sunset. What are the views of scholars about this: Do they need only to fast that day as qada' or is a kafara necessary ...?