What is the ruling on voluntary fasting before making up obligatory Ramadan fasts?

Scholars have differed concerning the ruling on observing voluntary fasts before making up missed obligatory fasts. Some scholars hold that it is not permissible and the one who does so incurs sin, because a supererogatory act should not be performed before an obligatory one. However, the majority of jurists permit it as long as there is no shortage of time, basing their argument on the fact that the time is flexible, similar to obligatory prayers which can be delayed within their prescribed time, and voluntary prayers are permissible during this period. Sheikh Ibn 'Uthaymeen chose this opinion, considering it the most evident and closest to what is correct, citing Allah's saying: ﴿And whoever is ill or on a journey - then an equal number of other days﴾ [Al-Baqarah/185], which indicates the flexibility of the matter and that immediacy is not a condition.