A separate fund or account shall be created in the city or county treasury for each improvement project and each such fund or account shall be identified by a suitable title. The proceeds from the sale of bonds and temporary notes and any other moneys appropriated thereto by the governing body of any city with a population of three hundred fifty thousand or more inhabitants which is located in more than one county, or the governing body of the county of the first classification without a charter form of government that has a population of at least one hundred sixty thousand inhabitants, or the governing body of any county of the first classification without a charter form of government containing a portion of a city with a population of at least three hundred fifty thousand inhabitants, or the governing body of any county of the first classification without a charter form of government as of August 28, 1995, that has a population of more than one hundred five thousand but less than one hundred twenty thousand inhabitants, shall be credited to such funds or accounts. Such funds or accounts shall be used solely to pay the costs incurred in making each respective improvement. Upon completion of an improvement, the balance remaining in the fund or account established for such improvement, if any, shall be credited against the amount of the original assessment of each parcel of property, on a pro rata basis based on the amount of the original assessment, and with respect to property owners that have prepaid their assessments in accordance with sections 249.941 to 249.945, the amount of each such credit shall be refunded to the appropriate property owner, and with respect to all other property owners, the amount of each such credit shall be transferred and credited to such city or county bond and interest fund to be used solely to pay the principal of and interest on the bonds or temporary notes and the assessments shall be reduced accordingly by the amount of such credit.