Tenant not paying increased rent according to agreement

My brother owns a shop in Mumbai. He had rented the shop to a bank 5 yrs back with an agreement mentioning that the rent will be increased every year. But the bank has not been paying the amount as per the agreement, rather the bank has been paying the starting rent for past 4 yrs. The bank says the person who signed the agreement is no longer working with them. But in fact the agreement is between my brother and bank signed by the bank employee on behalf of the bank. The agreement ends next month, Feb2019. How do my brother recover the rent from bank???