Is it permissible for the wife to save a portion of her husband’s money (the monthly allowance) without his knowledge, with the aim of contributing to improving the family’s standard of living, knowing that the husband is not stingy but does not care for saving for this purpose?

A wife is not permitted to take anything from her husband's personal money except with his permission, based on the Prophet's (peace be upon him) saying: "A woman should not spend from her husband's house except with his permission." If she saves something from it without his knowledge, she must inform him. Giving her an allowance does not permit her to dispose of it except in a reasonable manner or as he has authorized her. However, if the surplus from the allowance is for household needs, there is no objection to it. But saving money to buy a car is not considered ordinary household expenses, and she must inform her husband about it. If the money is her own personal allowance, she may dispose of it as long as it is permissible and without extravagance. The custom (urf) is the reference in all of this, based on the Prophet's (peace be upon him) saying to Hind: "Take from his money what is customary, sufficient for you and your children."