The trustees of trust funds of the city shall have custody of all capital reserves. Said trustees shall give bond in such amount and in such form as the city council or board of aldermen shall prescribe, and any trustee who shall make payment of income or principal from any such capital reserve fund before the approval of his bond in writing by the city council or board of aldermen shall be personally liable to the city for any loss resulting from such payment, to be recovered by the city at the suit of any citizen. The expenses of said trustees in said capacity and the expense of their bonds shall be charged as incidental city charges.