All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
There is nothing wrong if the husband discontinues the voluntary fast for the reason mentioned in the question or for other reasons. If a voluntary fast is initiated by the servant, it is only desirable, not obligatory, for him to complete it. If he breaks his fast, he does not sin, and he does not have to make it up as per our Fatwa. The Fiqh Encyclopedia reads:
The Shafi’is and Hanbalis are not of the view that completing a voluntary fast is an obligation, nor do they deem it mandatory to make up for a vitiated voluntary fast. This is based on ‘A’ishah’s report in which she stated that Hais [some food] was given to her. The Prophet asked her to bring it and he ate, although he had been fasting. An-Nasaa’i added in his narration that the Prophet’s said, “The likeness of a voluntary fast is like that of a man who allocated some of his wealth to give in charity; if he wishes, he may go ahead and give it; and if he wishes, he may keep it."
In another Hadith, Umm Hani’ narrated that the Messenger of Allah came to her and asked for some drink, and he drank. Then he offered it to her and she drank it. Then she said: “O Messenger of Allah! I was fasting.” The Messenger of Allah said: “The one fasting a voluntary fast is the trustee upon himself; he may either continue fasting or break the fast.”
According to the Hadith of Abu Sa’eed Al-Khudri, he narrated: I made some food for the Messenger of Allah and he came along with his Companions. When the food was served, a man said: ‘I’m fasting.’ Thereupon, the Messenger said: “Your brother has invited you [for a meal] and has taken the pain for your sake.” He then said to him, “Break your fast now and make up for it on another day, if you will.”
Allah knows best.