Wrong Information In Offer Letter

I joined a hyderabad based lifesciences company from my campus placement as area manager for west region. The offer letter which they offered me mentioned that My posting will be at their corporate office in Mumbai....but the truth is that company never had & still now doesn't have any such office in mumbai. They have only one office in Hyderabad since their inception. Please suggest that is this illegal.

Please confirm with the HR of the said company, it may be a typing error or they must have a corporate office in Mumbai which you or your resources may not be knowing/updated.
Further more if your posting is as Area Manager for western region they may be planning for a corporate office in Mumbai. As such for sales/field jobs, offices is not mandatory.
Let seniors also comment on this

Really amazing that your query is of dated Sept 06, 2013 and you have already joined the company past 4 months, then why was the query posted if it is of no relevance

Tell me, what exactly is your problem ?
What difference will illegality or otherwise do ?
Do you wish to stay in or leave ?
If you wish to leave, what is stopping you ?
No sane person thinks of taking legal action against an employer for just a difference between what was written in the letter and where the office actually is. What is the real problem ?