Question: Is it permissible to observe voluntary Sawm (Fast) with two intentions: the first to make up for missed days of Ramadaan and the second to apply an act of Sunnah (supererogatory act of worship following the example of the Prophet)? Is the Sawm of a traveller or an ill person who can fast valid or not?
Answer: It is not permissible to observe voluntary Sawm with two intentions: one to make up for missed fasts and another to perform an act of Sunnah. It is better for a person who is travelling a distance over which it is permissible to shorten Salaah (Prayer) not to observe Sawm. However, it is unobjectionable to observe Sawm in such a case, which will discharge the obligation. It is better for an ill person, if Sawm will be too hard for them, not to observe Sawm.
Furthermore, if it is certain or probable that Sawm will cause harmful effects or results in death, it will be obligatory to refrain from Sawm so as to avert harm and hardship. Later on, a traveller or an ill person must make up for the days of Ramadaan they have missed. However, if they observe Sawm despite difficulty, it will discharge the obligation… read more here.