Make up missed fasts or fast 6 days of Shawwal?

Question: A man owed expiation of fasting two months consecutively and he liked to fast the six days of Shawwal. So, is that permissible for him?

Answer: What is obligatory is to make haste in fasting the fast of expiation. Therefore, it is not permissible to fast the six days before it, because it (the six days of Shawwal) is voluntary while the expiation is obligatory and it must be done immediately. Thus, it is obligatory to fast it first before fasting the six days (of Shawwal) or any other voluntary fasts.

Shaykh `Abdul-`Azeez Bin Baz

Fataawa Islamiyyah, DARUSSALAM, vol.3 p.338 bin baaz