All grants approved and paid by the local boards of education shall be paid out of existing and available school funds allocated to such local systems by the state for educational purposes and from educational funds derived from local sources. The local system shall receive a refund relative to the amounts paid out in education grants in the amount and under the formula provided for in this Code section. The local board shall share in the cost of the education grant in the same proportion as it shares in the cost of the Adequate Program for Education in Georgia for such local system. The amount arrived at under such formula shall be paid by the Department of Education to the local system at the time and under the procedure prescribed by the State Board of Education. The state board shall include in its estimate of financial requirements to be filed with the Office of Planning and Budget, as required by law, an estimate of the amount of funds which may be needed during the next appropriations period for the purpose of paying the state's portion of the grants provided for under this article. The General Assembly shall make adequate provision therefor in appropriating funds to the Department of Education. The local school systems may, at their election, utilize local education funds to supplement the amount of state funds and the local funds paid under the formula set out in this Code section. Any such supplemental funds used shall be expended in the amount and under the procedure prescribed by the local board.