Overpaid salary

The client worked in a multinational company. The company has overpaid salary to client. 2 years after the client has left the company, the company does the final settlement and asks for refund of the excess salary paid. The question is that initially when the company paid the excess salary, tax was paid on the same. Now that when the client pays the excess salary amount, how can he claim refund of tax on the excess amount paid 2 years before? Also the client is currently not a salaried person.