The county boards of the several counties in this state are hereby authorized, when they deem the same expedient, to offer such rewards as in their judgment the nature of the case may require for the detection or apprehension of any person charged with or convicted of a felony, and pay the same, together with all necessary expenses, not otherwise provided by law, incurred in making such detection or apprehension, out of the county treasury; Provided, in no case shall the amount paid out for expense exceed the sum of three hundred dollars.