A grant under this chapter to meet project costs not permanently financed prior to July 1, 1969, shall be payable in one lump sum forthwith after the completion of such projects and a determination by the Commissioner of Education of the amount of such grant, but only if the application for review of preliminary plans and specifications on Form 2A for such project was submitted prior to October 1, 1975, in the case of towns and prior to October 15, 1975, in the case of regional school districts. The Commissioner of Education is authorized on behalf of the state, subject to the approval of the State Bond Commission, to make a commitment for such grant at any time prior to the completion of such project and to make advances thereon at such times and in such amounts as it shall deem advisable, provided the aggregate of such advances shall at no time exceed the estimated amount of such grant as determined by the Commissioner of Education. If the aggregate of such advances exceeds the amount of such grant as finally determined by the Commissioner of Education, the town or regional school district receiving such advances shall promptly repay to the state the amount of such overpayment.