The treasurer of each county of class one shall set aside the prosecuting attorney’s fees to be used as a contingent fund for the prosecuting attorney for the payment of the incidental expenses in bringing parties and witnesses from other states or countries and in properly preparing cases for trial, attending trial on changes of venue, attending at the taking of depositions, in printing briefs, and appearing before the appellate courts of the state, and generally the expenses he is put to in the proper and vigorous prosecution of the duties of his office. The fund shall be paid out as needed to the prosecuting attorney by the county treasurer, not exceeding twenty thousand dollars in any year, upon warrant of the prosecuting attorney approved and signed by a judge of the circuit court of said county. At the end of each year the county treasurer shall pay into the general revenue fund of the county any balance in his hands from fees, so collected, exceeding the sum of two thousand five hundred dollars.