Making up missed Ramadaan fasts is obligatory, and it is not permissible to break such a fast except in case of necessity. Once a person starts to observe a fast to make up a fast he missed, he is obliged to complete it, and it is not permissible for him to break it except for a legitimate shar’i excuse.
It was narrated that Umm Haani’ (may Allaah be pleased with her) said: “O Messenger of Allaah, I broke my fast when I was fasting.” He said to her: “Were you making up a missed fast?” She said: “No.” He said: “Then it does not matter if it was a voluntary fast.” Narrated by Abu Dawood, no. 2456; classed as saheeh by al-Albaani. This indicates that it would matter if she broke her fast when observing an obligatory fast. What is meant by “it would matter” is that it would be a sin.
With regard to what happened in your case, the expiation of intercourse is only required if what is broken is a fast in Ramadaan itself. Based on this, you do not have to do anything except make that day up again, as well as repent to Allaah and resolve not to do such a thing again.
Ibn Rushd said: The majority are agreed that no expiation is required for deliberately breaking a fast observed to make up a missed Ramadaan fast, because it does not come under the same category with regard to sanctity of time, i.e., Ramadaan.
Bidaayah al-Mujtahid, 2/80.
Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) was asked about a woman who broke a fast she was observing to make up for a missed Ramadaan fast, to make her guests feel comfortable. He replied: If she was making up a missed obligatory fast, such as making up a day of Ramadaan, then it is not permissible for anyone to break such a fast except in cases of necessity. With regard to breaking the fast because of the arrival of guests, this is haraam and is not permitted, because the basic principle in sharee’ah is that “Everyone who starts an obligatory duty is required to complete it, unless he has a legitimate shar’i excuse.” But if he is making up a voluntary fast then he does not have to complete it, because it is not obligatory.
Based on this, if a person is observing a naafil (supererogatory) fast and something happens that means he has to break his fast, he may do so. This is what was narrated from the Prophet (peace and blessings of Allaah be upon him), that he came to the Mother of the Believers ‘Aa’ishah (may Allaah be pleased with her) and said: “Do you have anything (to eat)?” She said: “We have been given some hays (a kind of meal).” He said: “Show it to me, for I started fasting this morning.” And he (peace and blessings of Allaah be upon him) ate some of it. This has to do with a naafil fast, not one which is obligatory.
Majmoo’ al-Fataawa, 20.