There is no sin upon him, but he must make up a day in place of it; because the voluntary fast, once started, becomes obligatory. If he breaks it, he must make up for it; as it was reported that the Prophet Muhammad (peace be upon him) said: "If one of you is invited, he should respond. If he is fasting, he should pray (i.e., make a supplication), and if he is not fasting, he should eat," in Sahih Muslim 2: 1054. If breaking the fast were permissible, it would have been better to break it in response to the invitation, which is the Sunnah. Aisha (may Allah be pleased with her) said: "Food was gifted to me and Hafsa while we were fasting, so we broke our fast, then the Messenger of Allah (peace be upon him) entered, and we said to him: O Messenger of Allah, a gift was sent to us and we desired it, so we broke our fast. The Messenger of Allah (peace be upon him) said: There is no harm upon you; fast another day in place of it," in Sunan Abu Dawood 2: 330, and Sahih Ibn Hibban 8: 284. Ibn Sirin fasted on the Day of Arafah and became extremely thirsty, so he broke his fast. He asked several of the companions of the Prophet (peace be upon him), and they instructed him to make up a day in place of it, in Musannaf Ibn Abi Shaybah 2: 290, and its chain is on the condition of the two Shaykhs except for Al-Taymi, as the four compilers narrated from him, and Ibn Sa'd, Ibn Sufyan, and Al-Daraqutni attested to him as mentioned in Al-Jawhar Al-Naqi 1: 315. See: I'laa Al-Sunan 9: 160. As for the Hadith of Umm Hani: "And if it is voluntary, then if you wish, make it up, and if you wish, do not make it up," Al-Tahawi said that it was uniquely narrated by Hamad ibn Salamah, and Abu 'Awanah, Qais, and Abu Al-Ahwas narrated it with the wording: "It does not harm you, and there is no problem," meaning that you are not sinful for breaking this voluntary fast, and there is nothing in that which negates that she must make up a day in place of it, as the Hadith of Samak is inconsistent. See: I'laa Al-Sunan 9: 158-159.