Reference: Fataawa Ramadhaan – Volume 2, Page 619, Fatwa No.620
Fataawa al-Lajnah ad-Daa.imah lil-Buhooth al-`Ilmiyyah wal-Iftaa. – Fatwa No.13475
Question: A man was making up a missed fast of Ramadhaan during Shawwaal of 1410AH when his wife, who was not fasting, presented herself to him and he was unable to control himself until he had intercourse with her. Provide us with a fatwa, and may you be rewarded.
Response: The one who broke his fast in other than Ramadhaan by engaging in sexual intercourse is required to make up that day in substitute for the day he broke his fast due to sexual intercourse, and there is no expiation required of him; This is because his engaging in sexual intercourse did not occur in Ramadhaan. [Additionally], he is required to repent to Allaah for what has occurred, and likewise, his wife is required to repent for that because she was the cause of him breaking his fast.
And with Allaah lies all the success, and may Allaah send prayers and salutations upon our Prophet Muhammad (sal-Allaahu `alayhi wa sallam) and his family and his companions.