The evidence from the Qur’aan and Sunnah and scholarly consensus indicates that the traveller is allowed not to fast during Ramadaan, then he should make up the number of days that he missed. Allaah says (interpretation of the meaning):
“and whoever is ill or on a journey, the same number [of days which one did not observe Sawm (fasts) must be made up] from other days”
If a person is in his hometown, then he decides to travel, he is not regarded as being a “traveller” until he has left the built-up area of his town. So it is not permissible for him to avail himself of the concessions granted to travellers, such as not fasting and shortening the prayer, when he has merely formed the intention of travelling. Rather Allaah has permitted the one who is travelling to break the fast, and he is not travelling until he has left his town.
Ibn Qudaamah said in al-Mughni (4/347), after stating that one who travels during the day may break the fast:
Once this is established, it is not permissible for him to break the fast until he has left the houses behind, i.e., he has gone past them and has left the buildings behind. Al-Hasan said: He may break the fast in his house, if he wishes, on the day that he intends to leave. Something similar was narrated from ‘Ata’. Ibn ‘Abd al-Barr said: The view of al-Hasan is odd; no one is allowed to break the fast when not travelling, and there is no evidence for that, either rationally or in the reports. A different opinion was also narrated from al-Hasan.
Then Ibn Qudaamah said: Because Allaah says (interpretation of the meaning):
“So whoever of you sights (the crescent on the first night of) the month (of Ramadan i.e. is present at his home), he must observe Sawm (fasts) that month”
The one who is present and sights the moon is not regarded as being a traveller until he has left the town. So long as he is still in the town, the rulings on those who are there and not travelling still apply to him. Hence he should not shorten his prayer. End quote.
Shaykh Ibn ‘Uthaymeen was asked about a man who intended to travel and broke his fast in his house, because he was unaware of the ruling, then he set out. Does he have to offer any expiation?
He replied: It is haraam for him to break the fast when he is in his house, but if he broke the fast before leaving his house, then he only has to make up the day. End quote, from Fataawa al-Siyaam, p. 133.
He said in al-Sharh al-Mumti’ (6/218):
The Sunnah and reports from the Sahaabah (may Allaah be pleased with them) indicate that if a person travels during the day, he may break the fast, but is it essential that he leave his town first? Or once he decides to travel and gets into his means of transportation is he allowed to break the fast?
The answer is that two views have been narrated from the salaf.
Some of the scholars are of the view that it is permissible to break the fast once he is ready to travel and there is nothing left to do but get into the means of transportation. They narrated that this is what Anas (may Allaah be pleased with him) used to do. But if you study the verse you will see that this is not correct, because up till now he has not started to travel and he is still a resident and non-traveller. Based on this, it is not permissible for a person to break the fast until he has left the houses of the town behind.
The correct view is that he should not break the fast until he has left the village. Hence it is not permissible to shorten the prayer until one has left the town, and it is not permissible to break the fast until one has left the town. End quote.
Based on that, if a person decides at night that he is going to travel, it is not permissible for him to start the day not fasting, rather he must form the intention to fast, then if he travels in the morning, he may break the fast after leaving his town.
Conclusion: if a person decides at night that he is not going to fast on the basis that he is going to travel the next day, he has made a mistake, and he has to make up that day, even if we assume that he did not travel, because he did not have the intention to fast from the night before, and the Prophet (peace and blessings of Allaah be upon him) said: “Whoever does not intend to fast from before dawn, there is no fast for him (i.e., his fast does not count).” Narrated by Abu Dawood, 2454’ al-Tirmidhi, 730; classed as saheeh by al-Albaani in Saheeh Abi Dawood.
In the event that he does not travel, he must refrain from everything that breaks the fast for the rest of the day, out of respect for the month, because he has broken the fast without a legitimate shar’i excuse.
See al-Sharh al-Mumti’, 6/209
The questioner has to seek Allaah’s forgiveness and repent to Him from what he has done, and he has to make up that day.
And Allaah knows best.