Interest earned from the investment of money collected under a special assessment under this act or of money received as bond proceeds from a bond issued under this act, or money from interest or penalties charged and collected on an unpaid special assessment under this act shall only be used for the following:

  (a) To pay for the improvement for which the special assessment is assessed.

  (b) To pay the principal and interest of bonds that are issued for the improvement for which the special assessment is assessed.

  (c) To pay the principal and interest of an advance from the township that is used for the improvement for which the special assessment is assessed.