Indeed, there are differences of opinion among scholars concerning this issue. Some of them state that, it is not permissible for one to offer voluntary fasting before making up for the obligatory missed days of Ramadan. Some others state that it is permissible for one to offer a voluntary fasting before making up for the obligatory missed fasts. Each person can choose the opinion that best suits him keeping in mind that Islam is the religion of easiness and mercy.
Responding to this, the eminent Muslim scholar, Sheikh Yusuf Al-Qaradawi, states:
Scholars have differed as to whether it is allowed for one with some missed fasts of Ramadan to offer a voluntary fasting before making up for these missed fasts. Some believe that it is not permissible for one to offer a voluntary fast before making up for the obligatory missed fasts. According to them, one must first observe the missed fasts and then offer the voluntary fasts one wants to offer. Some others are of the view that one may offer voluntary fasts before making up for the missed fasts one is obliged to observe.
In my point of view, the above second opinion preponderate over the first one because it was reported that `A’ishah, Mother of the Believers (may Allah be pleased with her), would make up for her missed fasts of Ramadan in the following Sha`ban. If so, what about offering voluntary fasts during the ten months in between? It would not be tenable that she would not offer voluntary fasts during these months. On the contrary, she was reported to have observed much voluntary fasting.
However, I see that one should first observe one’s missed fasts of Ramadan and then offer any voluntary fasts one wants to observe, for one does not guarantee how long one may live.