(a)he shall use all despatch in ascertaining whether or not the insecticide or its sale, distribution or use contravenes any of the provisions of section 18 and if it is ascertained that the insecticide or its sale, distribution or use does not so contravene, forthwith revoke the order passed under the said clause or, as the case may be, take such action as may be necessary for the return of the stock seized;