No, they are not required to pay the "back pay" unless you had a written contract including a union or collective bargaining contract governing your employment which requires you to be paid in accordance with the pay scale.  If you have a contract, you could sue for breach of contract to get the back pay though since suing your employer is a drastic step, it's not necessarily worthwhile doing so.Otherwise, without a contract, there is no obligation for the employer to pay you as per the pay scale--that's simply their voluntary, unilateral internal policy. And the law does not enforce non-contractual employer policies.