If a person becomes Muslim on the last day of Ramadan, does he have to pay sadaqat al-fitr?.
Yes, he has to pay sadaqat al-fitr, because he is now one of the Muslims, and in the hadeeth of Ibn ‘Umar (may Allah be pleased with him) it says that the Prophet (peace and blessings of Allah be upon him) enjoined zakaat al-fitr, a saa’ of dates, or a saa’ of barley, upon all the Muslims, slave and free, male and female, young and old. End quote.
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him).