Any person required to become a member of the retirement system pursuant to section 2 of this act shall receive credit for all of his previous service in the employ of the city provided that payments are made by such person in an amount or amounts calculated in accordance with the rules of the commission as may be necessary to provide the entire actuarial cost of the previous service credit. In the event that the person retires before he completes the payment for all of his previous service credit, credit for his service shall be given in direct proportion as the amount paid bears to the total amount of the obligation.
N.J.S. § 43:13-22.59a