(a) The construction and maintenance fund consists of money, effects, property, and proceeds received by the district from any source except that portion of tax collections necessary to pay principal and interest on bonded indebtedness.
(b) The interest and sinking fund consists of that portion of tax collections necessary for paying principal and interest on bonded indebtedness, and this fund may be invested for the benefit of the district as provided by law.
(c) Each fund shall be held for the purpose for which it was created, and if money is improperly paid from either fund, the board may transfer money in the two funds to restore the fund which was improperly used.