Is it permissible to give out a portion of money with the intention of receiving the reward for voluntary fasting, given the negative impact of fasting on one's mood, physical condition, and work performance?

If fasting negatively impacts a Muslim, they will be rewarded for their intention and resolve, and they should substitute it with other acts of worship for which they have energy, such as charity and spending money on good causes. Indeed, Ibn Mas'ud used to fast less because it prevented him from reciting the Quran. The best fasting is that which does not weaken the body from fulfilling the obligatory rights of Allah or His servants; otherwise, it is better to forgo it. Money is given with the intention of charity, not as a substitute for fasting.