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.
According to the most preponderant opinion of the scholars, if a husband gives both his wives what is considered enough in regard to accommodation, spending and clothing, then it is permissible for him to favor one of them over the other with some gifts; for more benefit, please refer to Fataawa 91728 and 91417.
Therefore, it becomes clear that it is permissible for you with greater reason to give your wife who helps you and voluntarily supports you in your business, a salary or the like in return for her help even if she did not ask you for that. By doing so, you would return her favor and reward her for it, and this is among the good morals of the righteous people.
Allaah Knows best.