What is the ruling on breaking the fast if one eats while doubting sunset has occurred?

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If a person eats while doubting the sunset has occurred, and this doubt remains unresolved, they must make up (qada') the fast. This is because the original legal presumption (Asl) is the continuation of daylight. If the person was certain the sun had set or that dawn had not broken when eating, and only became doubtful after eating without resolution, no makeup fast is required. This is analogous to one who performs prayer based on their best effort (Ijtihad) and subsequently doubts the correctness of their action after finishing the prayer.