The Halacha is that a married woman may give a small amount of her husband’s money to Tzedaka without his knowledge. We can assume that the husband allows her to do so. The size of this donation depends on the financial standing of the household.
However, if the husband specifically said that he does not want her to make any donations, then she can’t give any money to Tzedaka. (Shulchan Aruch 248:4)
Aruch Hashulchan (YD 248: 11, 13) rules that in a society where Bais Din is unable to compel people to give an appropriate amount of Tzedaka, a wife (of someone who gives less than halachicly required) is permitted to be the one to force her husband to give by doing so without his knowledge. However, Aruch Hashulchan emphasizes that this can only be done based on the directive of the local Rav, who determines that according to this person’s wealth we would force him to donate this amount.
Shevet Halevi (Vol.5 132:7) disagrees with this ruling. He references the Halacha (YD 148:1) that even when Bais Din forces a person to give Tzedaka, they may only take his property in his presence, with his knowledge. Surely the wife, even if she is acting on the directive of the Rav, has no more authority than Bais Din.