Airtel — wrong billing of data provided under surprise offer

I am a postpaid customer. On 30/4/17 i got sms from customer care about free data 0f 2gb for 3 months from may - july 17. Now i am getting e biil 0f amount equal to 20 times of normal rate that to data offered under surprise offer. I had a talk with customer care team and they agree for their mistake but not able to roll back stating that sim is provided by company i belong. I therefore request you to resolve the issue to overcharging me as the amount is deducted from my salary. I am attaching snapshots of offer sms and wrong e bills.