How to fight against the owner by keeping the job safe

This is case of "Salary cheating". Simply, The amount calculated from joining to till date by the appointment letter is less that I have received in my account. "Namaskar". I am working in an private Institution from august2009 still continuing. Salary is through bank. So bank statement is available. In my appointment letter the salary is mentioned as 20,000.( As per the post ) Appointment letter is given in the month July2009. But the owner give 10,000/- from Aug2009 to Sept10. After requesting many times they give 15000. from sept2010 to Sept2011. From sept 2011 to oct2012 they give 20000. In the year they revise every body's salary. and mine they do 25000. But do not give any written paper. After some month again they give 15000 till today. Owner say to disbursed the remaining amount ( difference )phase by phase. But don't act on it. If I will leave the job then what about my remaining money.It simply vanished. Other suggest: Leave the job and do any where-else. Owner also say if amount is not sufficient then leave, But why? it is my survival case. Am I working for the owner or for the organization. Sir,I passed the interview, sustain in the job for last 7 years.Why should I leave the job? Is it not any help from the law to solve the situation. Please Donot say : If U do not Leave then suffer". Waiting for the right path to fight keeping my job in safe.