Is it permissible for a wife to give Zakaat/Sadaqah on her husband's behalf, with his own money but without his knowledge? Or must it be with his knowledge and consent.
All Praise be to Allaah, The Lord of the worlds, I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
When the wealth of the husband reaches its Nisaab (i.e. minimum amount for which Zakat becomes obligatory; equal to 85 grams of gold) and a full Hijri year has passed, it becomes obligatory upon him to give Zakat immediately. If he refuses to pay Zakat, while denying its obligation he has disbelieved if he knows its ruling, and his marriage invalidated.
If he refuses to give Zakat due to stinginess, his marriage is valid. It is not right (valid) for his wife to Pay Zakat on her husband’s behalf without his knowledge and consent because the intention is a condition for Zakat to be accepted. If she pays Zakat on his behalf without his intention it is not accepted from him.
The wife has to do her best to advise him with kindness and leniency and remind him of Allaah, Most High, and His punishment because refusing to pay Zakat is one of the great major sins.
The Messenger of Allaah said: "If one is granted wealth by Allaah and does not pay the Zakat of his wealth, then on the Day of resurrection his wealth will be made like a bald-headed, poisonous, male snake with two black spots over the eyes (or two poisonous glands in its mouth). The snake will encircle his neck and bite his cheeks and say, ‘I am your wealth, I am your treasure.'" [Al-Bukhari and Muslim]
Likewise, spending in charity on behalf of the husband without his knowledge and consent is more deserving to be prohibited, because it is dealing with the wealth of others without their permission, unless the amount is insignificant and it has become customary among the people to tolerate such spending; then she can do it without his permission, and they will both be rewarded.
Allaah knows best.
*Helped by fatwas of islamweb.net