Airtel — aadhaar linking with mobile

Address:

Moradabad, Uttar Pradesh, 244001

I visited an airtel store in moradabad to get my mobile link with my aadhaar card. The staff there told me to visit a retail shop instead. When i visited the retail store i was charged rs. 20 for linking the aadhaar with mobile there. When i objected i was told that this is the rate with which it is being done if i want to do it, or else my number will be deactivated as per government rules. It is supposed to be without charge as per my information. What kind of scam is this? Please check and confirm if there is an official charge.