All perfect praise be to Allaah, the Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
The expiation for having intercourse during the day in Ramadan was explained by the Prophet, sallallaahu ‘alayhi wa sallam. It was narrated on the authority of Abu Hurayrah that he said, "While we were once sitting with the Prophet, sallallaahu ‘alayhi wa sallm, a man came and said, ‘O Messenger of Allaah, I am ruined.’ The Prophet, sallallaahu ‘alayhi wa sallam, asked: ‘What is it?’ He replied: ‘I had sexual intercourse with my wife while I was fasting.’ The Messenger, sallallaahu ‘alayhi wa sallam, asked him: ‘Can you afford to manumit a slave?’ He replied in the negative. The Prophet, sallallaahu ‘alayhi wa sallam, asked him: ‘Can you fast for two successive months?’ He also replied in the negative. The Prophet, sallallaahu ‘alayhi wa sallam, asked him: ‘Can you afford to feed sixty poor persons?’" [Al-Bukhari and Muslim]
Expiation by fasting cannot be made during the month of Ramadan because its days are already occupied with the obligatory fasting. The person who is required to make expiation should do it as soon as possible after Ramadan finishes. Expiation cannot be in Ramadan unless it is by means of giving food to the poor and the needy or freeing a slave.
Allaah Knows best.