Section 6A. A health maintenance organization may only enter into a group health maintenance contract with an employer if the group health maintenance contract is offered by that employer to all full-time employees who live in the commonwealth; provided, however, the employer shall not make a smaller health insurance premium contribution percentage amount to an employee than the employer makes to any other employee who receives an equal or greater total hourly or annual salary for each specific or general blanket policy of insurance for all employees. Notwithstanding the foregoing, a health maintenance organization may enter into a group health maintenance contract with an employer that establishes separate contribution percentages for employees covered by collective bargaining agreements.