A separate fund or account shall be created in the city treasury 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 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 section 67.463, 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 the 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.