Reference: Fataawa as-Siyaam libni Jibreen – Page 126, Fatwa No.150
Question: Whoever has [fasting] days remaining to make up from Ramadhaan and has not yet made them up, is it permissible for him to fast the voluntary fasts even though he has not made up the fasts remaining from Ramadhaan?
Response: It is not permissible for a Muslim to fast voluntary fasts whilst having obligatory fasts remaining [to make up]. So it is not permissible for him to fast with the intention of a voluntary fast whilst he has not made up that which he missed in Ramadhaan.
