How to legally claim due salary

I work for a local IT company, no Employment Certificate was issued but I do have some salary certificates from employer. During a particular project due to a delay in delivery the client cancelled the contract and this resulted in financial losses for our company, all employees were asked to voluntarily agree on a delay of few months, but delay was prolonged to 7 months for me, and the amount is till pending even after 5+ years. Our team has now been outsourced by another company, we get salary regularly but in cash. How to claim the due salary if the employer disagrees when asked for same?