In our society, Sadaqah al Fitr is given on behalf of the wife by her husband, and it is generally understood in our society that the wife has granted her husband permission to do so. Therefore, in order to give Sadaqah al Fitr on behalf of one's wife, it is not mandatory to explicitly ask for her permission.
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دلائل:
رد المحتار: (باب صدقة الفطر، 363/2، ط: دار الفکر)
ولو أدى عنهما بلا إذن أجزأ استحسانا للإذن عادة أي لو في عياله وإلا فلا۔
Allah the Almighty knows best.
Darul ifta Al-Ikhlas, Karachi.