The officer in charge of the finances of the city shall deduct from the salary of each firefighter participating in the plan provided in this chapter an amount to be determined by the bylaws of the firefighters relief association but which may not be less than five percent of the monthly salary of a first-class firefighter, until those firefighters have completed thirty years’ service with the department at which time the amount deducted may be decreased to a lesser amount to be prescribed by the bylaws of the relief association, but which may not be less than two and one-half percent of the monthly salary of a first-class firefighter. This amount must be paid to the secretary-treasurer of the relief association monthly and must be credited by the association to each firefighter individually in the state fund. All moneys paid into city pension funds prior to July 1, 1959, by firefighters participating in the plan provided for in this chapter must be paid to the secretary-treasurer of the association and credited by the association to the individual member in the state fund.