If bonds are issued, a separate public improvement district fund shall be created in the municipal or county treasury for each district. Proceeds from the sale of bonds, temporary notes, and time warrants, and other sums appropriated to the fund by the governing body of the municipality or county shall be credited to the fund. The fund may be used solely to pay costs incurred in making an improvement. When an improvement is completed, the balance of the part of the assessment that is for improvements shall be transferred to the fund established for the retirement of bonds.