Is it permissible to observe voluntary fasts while falling short in performing professional duties, or is it permissible to offer fidyah instead of fasting on Mondays and Thursdays?

Voluntary fasting is supererogatory, not obligatory, and breaking it is permissible with or without an excuse. If a Muslim intends to observe a voluntary fast but is prevented by an excuse such as illness, it is hoped that Allah will record the reward for his intention, based on the Prophet's (peace be upon him) saying: "If a servant falls ill or travels, what he used to do when he was healthy and at home will be recorded for him." No expiation by feeding is prescribed for inability to observe a voluntary fast, and it is not permissible to be remiss in one's work due to fasting.