Indebtedness not exceeding in the aggregate the sum of ten million dollars, heretofore or hereafter contracted by the city of Buffalo or the city of Rochester and indebtedness not exceeding in the aggregate the sum of five million dollars heretofore or hereafter contracted by the city of Syracuse for so much of the cost and expense of any public improvement as may be required by the ordinance or other local law therein assessing the same to be raised by assessment upon local property or territory.