What is the ruling on buying clothes for my wife (an un-Islamic hijab) with my own money?

It is not permissible for a husband to buy clothes for his wife that do not meet the conditions of the Shariah hijab if she wears them when going out or in front of non-mahram men, because that would be cooperation in sin and aggression.

Similarly, it is not permissible for him to let her buy such clothes herself, even with her own money, because that would be a dereliction of trust and guardianship.

However, if she only wears these clothes when she is alone with him, there is no harm in him buying them for her or in her buying them.