Maharashtra Natural Gas [Mngl] — arrears in first bill without explicit authorizations

We have recieved the first bill for period 06.10.2018 to 08.01.2019 for.

Meter # xxxxxx38482
Bp : xxxx4724
Bill # : xxxxxx5679

We were told at the time of installation that there is no cost to us so we gave our aadhar cards
We have received the first bill with arrears of 5900... We are really confused...
A) how can we have arrears in first bill? This cannot be installation as it was supposed to be free
B) the customer care support say it is security deposit and is refundable..
How can an refundable deposit be collected as arrears. Please use clear terminology

At this costing we will need 3 year to recover from lpg... So are thinking to of cancelling this service