It's illegal for an employer to pay less than minimum wage, but what if it's what the employee wants them to do?

I can see why it’d be illegal for an employer to withold the minimum amount from an employee who wanted/needed it, but what if it was the employee’s idea to be paid less than minimum wage? What if it caused no hardship for the employee? Does the law allow an exemption for cases like that, or would it still be considered a crime?