The observer of voluntary fasting is his own leader. That is why he is permitted, if he so desires, to abort the fast. This is the verdict of many scholars relying on the evidence of the following citations.
Umm Hanee (raa) related that "The Messenger of Allah entered into my quarters the day that Makkah was conquered. He was offered water to drink. He drank and passed it to me. I told him I was fasting! The Messenger (saas) instructed. "The observer of a voluntary fast is the leader of himself. If he wishes, he can continue the fast, and if he wants he can break''. (Ahmad)
Elsewhere, Abu Sa'eed Al-Khudri (raa) said, "I invited the Messenger of Allah (saas). He came with his companions. When the food was brought, a man refused to eat. He said, `I am fasting.' The Messenger of Allah (saas) commented: `Your brother invited you and he has taken the trouble and incurred the cost for your sake. Break your fast and make it up with another day if you want.'" (Bayhaqi)
These two ahadith are evidence that the voluntary fast is different from the mandatory fast in that the observer is allowed with or without a valid reason to abort the obligation of fasting. However, the second report shows also that it is desired to make up the aborted day.