He travelled during Ramadaan to a country where the people started Ramadaan on a different day

Praise be to Allah.

What matters when a person started fasting in one country and finished fasting in a country to which he travelled, is that if the total number of days that he fasted was twenty-eight, then he must make up one day, because the lunar month cannot be less than 29 days. If he has already fasted thirty days but the people in the country to which he has travelled still have one more day to fast, for example, then he must fast with them so that he may break the fast with them on the day of Eid, and offer the Eid prayer with them.