A revolving fund shall be disbursed only by checks or drafts, signed by at least two members of the board, or by some person who is authorized to do so by unanimous vote of the board. No person shall be so authorized until the person has filed with the board a bond in the amount required by the board and in no event less than double the amount of the fund. The bond shall be conditioned as security for the safety and proper disbursements of the fund.