Leave expired and extension refused because of error in cos

[lindalaila]

My application for tier 2 extension has been rejected because an inappropriate salary was mentioned on my cos. I had informed my employer about salary requirements and my salary had been adjusted accordingly (my last payslip proves this) but the change has not been reflected on the cos (which had been assigned before the salary update). My application has been rejected and the reason is the fact that the salary mentioned in my cos doesn't match the required salary for my profession.

My current leave has also expired 3 weeks ago and I am very confused at the moment because I don't know what step I should take next. From my understanding of the refusal letter, it's either I apply for an appeal or I get removed from the country immediately. Since it was my sponsor's responsibility to provide the right information on the cos (he is not very experienced with the system yet), I don't know if I stand a chance at the appeal.

Could you please give me some advices about what to do next?
Thank you in advance.