If upon retirement any compensation be granted to a person who has not been a contributor to the fund, as herein provided, for an aggregate period of 20 years, such person shall be required to pay to the Board for the benefit of the fund monthly an amount equal to the same per centum rate when he separated from the service of his compensation until such time as his contribution shall have been extended to a period of 20 years which shall be deducted from his semimonthly payment.