He has to fast for a vow; is it permissible for him to fast it with Ramadaan?

The vow must be fulfilled
as an act of obedience and righteousness, such as when a person vows to fast
one or more days, because the Prophet (peace and blessings of Allaah be
upon him) said: “Whoever vows to obey Allaah, let him obey Him.” Narrated by
al-Bukhaari (6318). 

If the vow mentions a
specific time, then it must be fulfilled at that time, such as if a person
vows to fast three days at the beginning of the month. But if it is general
and not specific, then it is permissible to fast it at any time, except
Ramadaan, the days of Eid al-Fitr and Eid al-Adhaa, and the days of
al-Tashreeq. 

With regard to Ramadaan,
the time is already occupied with the obligatory fast, and it is not valid
to put any other fast with it. With regard to the days of Eid and
al-Tashreeq, it is forbidden to fast on these days. Al-Bukhaari (6212)
narrated that Zayd ibn Jubayr said: I was with Ibn ‘Umar, and a man asked
him: I vowed to fast every Tuesday or Wednesday as long as I live, but this
day coincided with the Day of Sacrifice. He said: Allaah has enjoined
fulfilling vows, and He has forbidden us to fast on the Day of Sacrifice.
The man repeated (his question), and he gave a similar answer, and no more.
 

Al-Haafiz Ibn Hajar said:
There is consensus that it is not permissible to fast on the day of (Eid)
al-Fitr or the day of Sacrifice (Eid al-Adha), whether it is a voluntary
fast or in fulfilment of a vow. End quote. 

Al-Bukhaari (1998) narrated
from ‘Aa’ishah and from Saalim from Ibn ‘Umar (may Allaah be pleased with
them all) that they said: No concession was granted allowing us to fast
during the days of al-tashreeq, except for the one who cannot find a
sacrificial animal. 

The scholars drew attention
to the fact that it is not permissible to observe any other fast in
Ramadaan. 

Al-Nawawi (may Allaah have
mercy on him) said in al-Majmoo’ (6/315): al-Shaafa’i and his
companions (may Allaah have mercy on them) said: Ramadaan is only for the
fast of Ramadaan, and it is not valid to observe any other fast at that
time. If a resident or a traveller or a sick person intends to fast as an
expiation or in fulfillment of a vow or a voluntary fast or a general fast,
his intention is not valid and his fast is not valid, and does not count
either for what he intended or for Ramadaan. End quote. 

Ibn Qudaamah (may Allaah
have mercy on him) said in al-Mughni: The traveller may not fast in
Ramadaan for something other than Ramadaan, such as a vow or making up a
fast, because not fasting is permitted to him as a concession to make things
easier, and if he does not want to avail himself of that concession, then he
has to do what is originally prescribed. If he intends to observe a fast
other than Ramadaan, his fast is not valid, either for Ramadaan or for what
he intended. This is the correct view, and it is the view of most of the
scholars. End quote. 

He also said (13/645): 

If he vows to Allaah that
he will fast for one month, and he intends to fast the month of Ramadaan in
fulfilment of his vow, that is not acceptable. Similarly, if he vows to pray
two rak’ahs, it is not acceptable for him to pray Fajr in fulfilment of his
vow and for Fajr prayer. End quote. 

Shaykh Ibn ‘Uthaymeen (may
Allaah have mercy on him) said: If a person makes a vow that is connected to
a condition, he must fulfil the vow when that condition is met and not delay
it. For example, if he says: “If Allaah heals me of this disease I will fast
for three days for Allaah,” and he recovers from that sickness, then he must
hasten to fast and not delay it, because Allaah says (interpretation of the
meaning): 

“And of them are some
who made a covenant with Allaah (saying): ‘If He bestowed on us of His
Bounty, we will verily, give Sadaqah (Zakaah and voluntary charity in
Allaah’s Cause) and will be certainly among those who are righteous’

76. Then when He gave
them of His Bounty, they became niggardly [refused to pay the Sadaqah
(Zakaah or voluntary charity)], and turned away, averse.

77. So He punished them
by putting hypocrisy into their hearts till the Day whereon they shall meet
Him”

[al-Tawbah 9:75-77] 

As for the vow that is not
connected to a condition, such as if a person wants to make himself fast for
three days, for no reason, then he must hasten to fulfil it, but it is not
like the first case. If Ramadaan comes and he has not fasted, then it is
well known that he should start with fasting Ramadaan, and when Ramadaan
ends he can fast in fulfilment of his vow. But if he fasts in fulfilment of
the vow in Ramadaan, neither his fasting in fulfilment of his vow nor his
fasting for Ramadaan are valid. 

If a man vows to fast for
three days, and he fasts three days of Ramadaan in fulfilment of his vow,
what should he do? His fasting did not count in fulfilment of his vow or for
Ramadaan. As for it not fulfilling his vow, that is because the time of
Ramadaan is restricted to Ramadaan, and he cannot observe any other fast at
that time. As for it not counting for Ramadaan, that is because he did not
form the intention to fast for Ramadaan, and the Prophet (peace and
blessings of Allaah be upon him) said: “Actions are but by intentions, and
each man will have but that which he intended.” End quote from al-Liqa’
al-Shahri (52/4). 

To sum up: The month of
Ramadaan is allocated specifically for the obligatory fast, and it is not
permissible to observe any other fast at that time, whether it is a
voluntary fast or a fast in fulfilment of a vow, whether one is travelling
or not. It is also not permissible to form a combined intention, by
intending to observe both the obligatory fast and the fast in fulfilment of
a vow at the same time, because they are two separate acts of worship which
cannot be done with one intention. 

Based on this, it is not
permissible for you to fast in fulfilment of a vow with Ramadaan. 

And Allaah knows best.