Praise be to Allaah.
If a Muslim
intends to fast one day and starts to fast, then he decides to break the
fast, he may do so, because completing a voluntary fast is not obligatory,
but it is mustahabb for him to complete it if there is no excuse. If there
is an excuse or an interest to be served by breaking the fast, there is
nothing wrong with doing so in this case, and the fasting person who broke
his fast will be rewarded – in sha Allah – for what he intended of serving a
shar‘i interest, such as if he wanted to be kind to a friend of his who
would be offending by his not eating food offered when visiting him or not
accepting his invitation, or if he wanted to do an act of worship that
fasting made him too weak to do, and it could bring him more reward than
observing the naafil fast, and so on.
But we have not
come across anything to indicate that he will be given the reward in full
like one who actually fasted; rather what seems to be the case is that he
will be rewarded for his intention, and the reward is according to the
intention; it is not the same as the reward of one who had the intention and
did the action.
For more
information please see the answer to question no.
49610.
Secondly:
The more correct
view is that the intention of breaking the fast to accept an invitation the
next day means that one cannot form the intention to fast that day.
Based on that, it
is not valid for one who has decided to accept an invitation to eat on a
particular day to form the intention of fasting on that day from the night
before.
Fasting does not
prevent one from accepting an invitation; rather it is permissible for one
who receives an invitation when he is fasting to offer du‘aa’ for the one
who invited him and not break his fast. Thus the fasting person will
maintain his habitual fast whilst also accepting his friend’s invitation.
But if he knows that the one who is inviting him has gone to some effort for
his sake and that if he does not eat when visiting him it will upset his
friend, then it is mustahabb for him to break his naafil fast and eat with
his friend.
And Allah knows best.