In proprietary and small private firms the final authority is the owner or the person who employed you.
In large establishments there will be a proper system and things will be put in black and white with certain e conditions binding both on all or specified contexts and situations. Usually such terms re given broadly in employment contracts, appointment letters or public advertisements and announcements . They will be again reminded by circulars, notices and direct letters. Any changes will be based on the earlier terms and any change will be intimated also.
In establishments where there are bipartite or tripartise systems in vogue like government sector, banks, etc the wage structure is amended by the pay commission, settlements etc as per the system in vogue after proper discussion and mutual acceptance.
As the particulars inyou case are not given, I guess either it may be a proprietary, partnership or private limited organisation. There is not much scope to oppose. The best solution is to quit. If it is a large organisation, then re-look into the job contract and take up with the employer if it is not in conformity with the contract. In other cases, discrepancy or violations may be taken up through the recognised trade union.
If not fitting into these, then, there would be some strong reason to impact your salary - probably some action inviting penal provisions.