Airtel — charging for aadhar linking with mobile number

Address: Kolkata, West Bengal, 700032

I went to airtel storein baghajatin station road, kolkata on 12/08/2017 at about 11.00 am for getting my phone account linked with aadhar. The person on duty asked me rs 20/- (Twenty), for linking.. Although i provided money as demanded, as i were not aware of charges. When i demanded a receipt he shouted at me that no receipt will be given. Recently i came to now the process is actually free? Then is it legitimate for airtel authorized stores to charge? What should i do now. Please advise