Idea Cellular — Giving wrong information

Address:

Pune, Maharashtra

I got a call from idea cellular company regarding discount of 75 rupees on every bill for one year if i pay them 450 rupees in advance.
I accepted the offer since i was saving 450 in one year.
Now when the bill has come its just showing few rupees as discount.When i contacted idea cellular customer care then they told that the discount is only on the usages nt on the whole bill.
So if i will not make any call in a month then i will not the get the discount on total bill.
I am having 199 post paid plan.
This hidden fact was not told i start.I was told told i will get discount on total bill.Now they are say that its on usages i.e call made during that month.
This is also not applicable if a person is on roaming.
I asked then to cancel the offer then they are not ready.
Please help me in getting my refund