I am really keen on observing voluntary fasting. Allaah is quite aware of my strong wish for this. However, when I fast, I suffer a headache that is so painful that I am not able to perform well at my job. Is it permissible to fast even though I will have to obviously neglect my job? Or may I pay money in charity instead of fasting Mondays and Thursdays that I am not able to observe?

All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.

Voluntary fasting is recommended, not mandatory. If you are able to observe voluntary fasts, then you may do so, and you will be rewarded for it. If you are not able to observe voluntary fasts, then you do not have to do so. If you start a voluntary fast and later feel too weak to complete the rest of the day, then you may break your fast. Since breaking an obligatory fast becomes permissible if one has a valid excuse, then breaking a voluntary fast with an excuse is permissible by greater reason. Rather, breaking a voluntary fast is permissible without an excuse.

If you would like to observe a voluntary fast and you are not able to do so because you are sick, for example, then we hope that Allaah, out of His grace, will give you the reward for whatever you would have done had you been able. The Prophet, sallallaahu ‘alayhi wa sallam, said: “When a person becomes sick or goes on a journey, then he will be rewarded for the good deeds that he used to perform when he was home and in good health.” [Al-Bukhari and others]

We have found no textual proof to support the permissibility of feeding in charity in place of voluntary fasting.

Finally, being negligent in one's job is impermissible under all circumstances. 

Allaah Knows best.