Any payment made by an employer to a member of any retirement system enumerated in RCW 41.50.030 based on either an agreement of the employee to terminate or retire; or notification to the employer of intent to retire; shall affect retirement as follows:

(1) If the agreement does not require the employee to perform additional service, the payment shall not be used in any way to calculate the pension benefit.

(2) If the agreement requires additional service and results in payment at the same or a lower rate than that paid for the same or similar service by other employees it may be included in the pension benefit calculation but shall be deemed excess compensation and is billable to the employer as provided in RCW 41.50.150.

(3) If the agreement requires additional service and results in payment at a rate higher than that paid for the same or similar service by other employees, that portion of the payment which equals the payment for the same or similar service shall be treated as described in subsection (2) of this section, and the balance of the payment shall be treated as described in subsection (1) of this section.