Praise be to Allaah.
Firstly:
Masturbation
is haraam; we have previously discussed the ruling on it and how to deal
with that in the answer to question
329.
Secondly:
The
expiation for breaking an oath is explained in the verse in which 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”
[al-Maa’idah
5:89].
So the
individual has the choice between three things: feeding ten poor persons or
clothing them or freeing a slave. If he cannot do any of these things, then
he moves to the option of fasting, so he should fast three days.
Thirdly:
If a person
is unable to do any of these three things, so he fasts three days instead,
then after that he has enough money to do one of these three acts of
expiation, his fasting is still sufficient and he does not have to resort to
any of these options. This is the view of the majority of scholars. So if he
had started the fast then things became easier for him and he could afford
to feed poor people after he had started the fast, he does not need to go
back to the option of feeding poor persons, so this is even more appropriate
in the case of one who fasted and has finished the fasts. Ibn Qudaamah said:
If he has started to fast, then he becomes able to free a slave or feed or
clothe poor persons, he does not have to go back to those options. That was
narrated from al-Hasan and Qataadah, and it was the view of Maalik,
ash-Shaafa‘i, Ishaaq, Abu Thawr and Ibn al-Mundhir. … Because it is an
alternative and it is not invalidated by becoming able to do the action to
which it was an alternative. So he does not have to go back to the first
option after starting it. Similarly, if a person who is doing Hajj tamattu‘
and cannot afford a sacrificial animal has started to fast the seven days,
he cannot change his mind about that, and there is no scholarly difference
of opinion concerning that.
End quote
from al-Mughni by Ibn Qudaamah, 10/22
Based on
that, your fast is sufficient as expiation and cancels out the oath. Then if
you repeat the act of masturbation which you have sworn not to do, you do
not have to offer expiation unless you had renewed the oath and uttered it,
then broken it.
And Allah knows best.