Section 34A. Notwithstanding the provisions of sections thirty-two and thirty-four, county commissioners may in any fiscal year enter into agreements with cities and towns for the allocation of funds for highway maintenance and construction in accordance with the provisions of chapters eighty-two and eighty-four for such year; provided, that the aggregate amounts so allocated by any county shall not exceed the amount appropriated for highways for such county in the appropriation act for the previous fiscal year; and provided further that any unexpended balance of such allocations remaining at the end of a fiscal year may be expended under such agreements until September thirtieth of the following fiscal year.