Good day,

The question as to whether the company may dismiss you is entirely dependent on the companies disciplinary policy.   The policy will dictate the severity of moonlighting outside of your normal working hours and what the appropriate punishment would be. Furthermore its seems as if you are happy with your income you derive from your moonlighting, if you feel that it is a more profitable endeavor, then it is entirely up to you to decide as to whether to resign. From what I understand about the employment relationship between an employer and a employee, The employee has a fiduciary duty not to be in direct competition with his employer and consequently to render his labour potential to said employer. To this, employers generally  would want their employees to hand their entire labour potential over to the employer, the fact that you have the opportunity to moonlight would mean that generally they are not utilizing your entire potential.

One thing that I should however bring up is that if you like your current job then you should not damage your employment relationship. This would mean complying with what your manager asks.  If you feel that the reduction from your salary is an unreasonable punishment, then I would advise you to approach a labour law attorney in order to assess this.  

LP