(o)"turnover" means the aggregate of the amounts of sale price receivable, or, if a dealer so elects, actually received by the dealer, in respect of any sale of goods made during any prescribed period in any year after deducting the amount of sale price, if any, refunded by the dealer to a purchaser in respect of any goods purchased and returned by the purchaser within the prescribed period: Provided that an election as aforesaid once made shall not be altered except with the permission of the Commissioner and on such terms and conditions as he may think fit to impose;
