Question 162: Is it permissible to break supererogatory fast i.e. such as Mondays and tuesdays due to severe hunger or fear of not being able to do a good job at work? With its source/proof thanks. What is the ruling on Permissibility of breaking a mustahab fast when invited?
Answer 162: When a person observe a mustahab fast it is not obligatory upon him to complete it. If one of his brother in faith has invited him to break his mustahab fast he would be canonically recommended to accept his invitation and break the fast.
Sayyid Sistani (ha) says: In fact, if one of his brethren-in-faith invites him to a meal, it is Mustahab that he accepts the invitation and breaks the fast (Mustahab fast) during the day time even if it may be after Zuhr.
If you break your mustahab fast you not deprive from getting the reward of fasting, furthermore if you don’t let your brother in faith that you are fasting and accept his invitation Allah (SWT) will give him the reward of a fasting for one year. Therefore, you are allowed to break your mustahabb fast anytime you want especially when you are faced with such problems.
Note: The meaning of the invitation of a brethren-in-faith is when he seriously invite you to a meal.
For further information on this matter, please read the following answers:
Index: Studying in the month of Ramadhan is not a valid excuse to escape fasting, answer 572.
 . Tawzihul Masael of maraja (with connotation), Sayyid Roohullah al-Moosavi al-Khomeini, question 1748. Ibid, Vol. 1, Pg. 985; Tawzih al-Masael (Imam Khomeini), Pgs. 362-363, issue 1657.
 . Biharul Anwar, Vol. 17, Pg. 206.