Fathwa - Ruling on combining the fasting of the six days of Shawwaal and fasting to expiate an oath

Question
Is it permissible to observe the fast of expiating an oath with the
six days of Shawwaal?
Answer
All perfect praise be to Allaah, The Lord of the worlds. I testify
that there is none worthy of worship except Allaah, and that Muhammad,
is His slave and messenger.
Some jurists mentioned that whoever fasts the sixdays of Shawwaal with
the intention of making up (a missed obligatory fast), vow or the like
– such as expiating an oath– he attains the reward of the six days of
Shawwaal but not completely.Al-Khateeb Ash-Shirbeenisaid: "Ifone fasts
in Shawwaal to make up (a missed obligatory fast), vow or the like,
does he attain the reward of the Sunnah or not? I do not know that
this matter was mentioned but, apparently, he attains it. However, he
does not attain the said reward, especially he who missed Ramadan and
made it upin Shawwaal because theaforementioned meaning does not apply
to him, (The Prophet's Hadeeth: "Whoever fasts in Ramadan and follows
it with six days of Shawwaal, then it is as if he fasted
perpetually.") This is why some scholarssaid that it is recommended
for him in this case ( if he could not fast the six days during
Shawwaal ) to fast six days in Thul-Qa'idah because it is recommended
to make up for Sunnah fasting. This applies if one adoptsthe view that
the reward is not attained but, if we said that it is attained – as it
appears as previously mentioned – then it is not recommended to make
anything up for them.
Whoever fasts for the sake of expiating an oathin Shawwaal, this
avails him of the obligatory fasting. According to what was stated
byAl-KhateebAsh-Shirbeenihe also attains the original reward of the
Sunnah of fasting six days of Shawwaal without attaining the entire
reward. Therefore,what is better and more preferable is to single
outeach of them with separate fasts to attain the promised reward for
sure.
Allaah Knows best.