Is it permissible to take a commission from equipment sellers without the employer's knowledge, if the employee has not yet started receiving his salary, or if the purchase contract will be signed after the employee joins the company?

If the husband performs these tasks for his employer, then it is not permissible for him to take a commission without the employer's knowledge, because it is forbidden for an employee to accept anything given to him because of his position. However, there is no objection to accepting what is given to him in return for a service or work he performed outside the scope of his employment.