All perfect praise be to Allah, the Lord of the worlds. I testify that none is worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
Foreplay is forbidden between the spouses in Ramadan during the daytime. They are considered to have broken their fast if they ejaculated semen. They have to make up for that day and repent to Allah Almighty.
The scholars differed concerning the one who breaks his fast deliberately in Ramadan without intercourse; should he offer an expiation besides making up for it or not? The majority of the scholars said that he is not liable to offer expiation.
Based on that, the questioner and his wife should make up for that day and repent to Allah Almighty from this sin. Concerning the expiation of intercourse, many people of knowledge held that the correct opinion is that it is obligatory upon them. However, some of them held that it is not obligatory upon them as long as they did not break the fast by intercourse; especially given that they did not try to avoid expiation by breaking the fast before intercourse.
Allah knows best.