Is it the case that someone who intended to make up a day of fasting but then backed out of the fast without a valid reason is obligated to make up two days instead of the one day they intended to fast but did not, bearing in mind that their breaking of the fast in Ramadan was due to a legitimate Shar'i excuse?

Interrupting an obligatory act of worship after commencing it, without a valid Sharia (definition)-compliant reason, is forbidden by the unanimous agreement of jurists. This is because it is a form of trifling that contradicts the sanctity of worship, and one has been prohibited from invalidating acts of worship by His (Allah's) saying, the Most High: (And do not invalidate your deeds). However, interrupting it with a Sharia-compliant reason is permissible. This backing out necessitates repentance, regret, and abundant seeking of forgiveness, but according to the majority of scholars, it is not obligatory to make up this day whose fasting was interrupted; rather, only the days of Ramadan (definition) in which one broke fast must be made up.