I was talking about an accountant.

You don't need to have a union to have an employment contract. A contract under the law is an agreement, it need not even be written down. He offered a job with certain pay and benefits and the employee accepted by taking the job on those stated terms. He violated that contract by not abiding by the agreed upon pay. That's actionable, no union needed.

All unions do nowadays is require the employee to pay them for something the employee can do for themselves. Closed shops and mandatory payroll deductions prevent workers from operating in their own best interest. If unions really had something valuable to offer they wouldn't need government to extract their dues and run their protection rackets for them, employees would voluntarily pay for the "benefits" they bring.