All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
It is not permissible for the wife to take anything from the money of her husband without his permission; the fact that the husband is not good in saving money or that she wants to save the money for the future needs, is not a sound reason to take anything from his money.
Instead of this, a wife should advise her husband in this regard in a soft and gentle manner while clarifying to him the importance of saving and its praiseworthy effects, as by doing so it is most likely that he will take heed.
Allaah Knows best.