To pay for any work done under RCW 85.06.640 through 85.06.700, or matters incident thereto, the commissioners of said district may use any money raised or to be raised by collection of any unexhausted balance of assessed benefits as theretofore established upon the lands of said district and/or by assessments for maintenance, levied as provided by law; or they may issue warrants of such district redeemable by levies which shall be added to the annual cost of the maintenance of said system and be paid from the maintenance fund from time to time; or they may combine such methods of payment.