(1) When a member employed on a part-time basis is eligible to purchase periods of full-time service or one-for-five service, the compensation used to calculate the cost to purchase the full-time or one-for-five service will be the actual part-time compensation earned.

(2) If the member later retires with a full-time final average compensation or highest average compensation, the member may either:

(a) have the amount of full-time service purchased under (1) proportionally reduced based upon the ratio of time worked when the service was purchased to full-time work; or

(b) retain the full-time service by paying the difference between the cost actually paid and the cost had the member been paid a full-time salary at the time of the purchase, plus interest equaling the actuarially assumed rate of return for the trust fund in effect on the date of the member's purchase.