What is the ruling regarding eating when invited while observing a voluntary fast?

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If the fast is voluntary, it is recommended for the invitee to eat, as they have the option to exit the fast. When eating serves to fulfill the invitation and bring joy to the host, it is considered superior. It is related that the Prophet, peace be upon him, encountering a fasting guest who withdrew, instructed him to eat, saying, 'Your brother invited you, and made an effort for you; eat, and then fast a day in its place if you wish.' It is permissible to complete the voluntary fast, but the person should supplicate for them and inform them of the fast so their excuse is known, thus clearing them of blame for not eating.