Question: If someone intends at night to observe Sawm (Fast) on the next day, but they break Sawm in the morning or any time due to some reason, should they make up for that day because they intended to observe Sawm and did not do so?
Answer: If it is a Waajib (obligatory) Sawm, such as Sawm of a vow or making up for a missed day in Ramadaan, it is impermissible to break Sawm. Breaking an obligatory Sawm without an excuse is a sin, andthey must make up for that day. However, if it is a supererogatory Sawm, it is better to complete Sawm. If Sawm is broken, there is nothing wrong in this and making up for it is not required... read more here.