Praise be to Allaah.
What the Muslim should do is protect his oaths and not swear
oaths a great deal or swear oaths except with regard to matters that are
deserving of oaths. Allah, may He be exalted, says (interpretation of the
meaning): “And protect your oaths (i.e. do not swear much)” [al-Maa’idah
Shaykh as-Sa‘di (may Allah have mercy on him) said in his
Tafseer: “And protect your oaths (i.e. do not swear much)” means
refrain from swearing by Allah falsely, or swearing oaths frequently, and
protect them if you do swear oaths by not breaking them, unless breaking the
oath is better.
Tafseer as-Sa‘di, 1/242
The expiation for breaking an oath (kafaarat yameen) is to
feed or clothe ten poor persons, or to free a slave. If that is not
possible, then one should fast for three days. Allah, may He be exalted,
says (interpretation of the meaning):
“Allah will not punish you for what is uninentional in
your oaths, but He will punish you for your deliberate oaths; for its
expiation (a deliberate oath) feed ten Masakin (poor persons), on a scale of
the average of that with which you feed your own families; or clothe them;
or manumit a slave. But whosoever cannot afford (that), then he should fast
for three days. That is the expiation for the oaths when you have sworn. And
protect your oaths (i.e. do not swear much). Thus Allah make clear to you
His Ayat (proofs, evidences, verses, lessons, signs, revelations, etc.) that
you may be grateful”
It is not permissible for you to move to the option of
fasting unless you are unable to do any of the three acts of expiation,
namely feeding poor people, clothing them or freeing a slave.
Ibn al-Mundhir (may Allah have mercy on him)said:
(The scholars) are unanimously agreed that if a person who
swore an oath is able to feed or clothe poor people, or to free a slave, it
is not acceptable for him to fast if he breaks his oath. End quote.
al-Ijmaa‘, p. 157
There is no reason why you should not observe naafil fasts,
such as on Mondays and Thursdays and so on, before observing fasts of
expiation, or before you have finished all of them, but that is on condition
that you do not regard them as part of the expiation.
But we advise you to hasten to observe the fasts of expiation
first, if you are unable to do the things mentioned above with regard to
offering expiation for breaking an oath. Hastening to observe the fasts of
expiation takes priority because it is obligatory and doing what is
obligatory takes precedence over observing naafil fasts.
See also the answer to question no.
And Allah knows best.