The said chief county detective shall receive a salary of six thousand five hundred ($6,500) dollars per annum, the said assistant chief county detective shall receive a salary of three thousand five hundred ($3,500) dollars per annum, and ten of the special county detectives shall receive a salary of two thousand five hundred ($2,500) per annum each, and ten of the special county detectives shall receive a salary of two thousand three hundred ($2,300) dollars per annum each, together with all necessary traveling expenses; which said salary and expenses, having been verified by affidavit of the chief county detective, assistant chief county detective, or special county detective, incurring the same, and approved by the district attorney, shall be paid out of the treasury of the county on a certificate issued by the district attorney, directed to the controller of the county, who shall order warrants for said amounts according to law. In the event of the appointment of a less number of special county detectives than twenty, the district attorney shall have the power to determine in which class, as to salaries, the appointee or appointees shall be placed.