Supplementary payment to retired teachers under Section 17-154 shall not accrue until such retired person has paid a total additional contribution of $5 per year for each year of validated teaching service, plus interest at 5% per annum from October 1, 1975. If retirement pension was not computed according to average salary as defined in Section 17-116, 1% of the monthly base pension multiplied by each complete year of service forming the basis of his service retirement pension shall constitute the total additional contribution.
     The supplementary payment (1) shall be prorated on a monthly basis as a one-twelfth addition to monthly payments due on the service retirement and disability retirement pensions, (2) shall begin on the date on which the payment of such allowance is next due after such contribution and interest thereon have been paid, and (3) shall continue to be paid only to the extent that funds are available in the Retired Teachers Supplementary Payment Fund established hereunder for this purpose; provided that in no case shall the present or future service retirement pension or disability retirement pension of any person be reduced hereby. No part of any such supplementary payment shall be an obligation of the fund otherwise established under this Article.