What is the validity of a fast intended as a voluntary fast (Tattau') during the month of Ramadan?

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If a person fasts the month of Ramadan while intending it as a voluntary fast, the fast is invalid, neither counting for Ramadan nor the voluntary intention. This is the correct view within the madhhab and is held by the majority of scholars. The primary evidence against allowing the fast to count for another obligation, even voluntary ones, is that the dispensation to break the fast is tied to the valid reason (the journey), and this dispensation cannot be used for an obligation other than the Ramadan fast itself.