To answer your specific question, yes an employer can so instruct its employees. It can also decide to pay everyone a different amount if it so desires, and it can pay a friend substantially more if it wishes. The only thing an employer cannot do is to pay different wages based on protected classes of people. For instance, paying men more than women because they are men, or paying Caucasians more than Hispanics because they are Caucasian.
That is the nature of the employment relationship. The employer has great freedom to treat people differently as long as the different treatment is not based on unlawful discrimination against a protected class of people.
I wish you the best of luck.
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