All praise be to Allah.

The scholars differed concerning the ruling on observing voluntary fasts before making up missed fasts.

Some scholars are of the opinion that “it is not correct to observe voluntary fasts before making up missed fasts, and that the one who does that is a sinner.” The reason for this is that an optional action should not be done before an obligatory action.

While as other scholars are of the opinion that it is permissible as long as there is time to fulfil all the missed fasts and the time is not short. They said that it is permissible to observe the optional fasts just like the optional prayers are performed before the obligatory prayers. For example, the time of Dzuhr starts after the sun reaches its zenith and ends when the shadow of an object is equal in length to its length, so one may delay Dzuhr to the end of its time, and during this period it is permissible to offer optional prayers, because there is sufficient time.
Majority of the fuqaha are of this opinion and this was the view favored by -Shaykh Muhammad ibn ‘Uthaymeen, who said: “This is the most correct view, and his fast is valid and he is not a sinner, because this is a clear analogy… And Allah says (interpretation of the meaning):

‘and whoever is ill or on a journey, the same number [of days which one did not observe Sawm (fasts) must be made up] from other days’

[al-Baqarah 2:185]

This means that he has to make up (the missed fasts) on other days, but Allah did not stipulate that they must be consecutive days; if He had stipulated that, then they would have had to be made up immediately. This indicates that there is some room for maneuver in this matter.”

And Allah knows best.