Reliance Fresh — unsatisfactory billing

Address:

Bangalore, Karnataka, 560083

I had purchased 40 packets nandhini slim milk and biller said that der is a discount of 300 on purchase of 20 packets.. So effectively i got 600 discount.. But store manager came and said this billing is incorrect and asked me to return all the item, and later i had stay for another hour to purchase the same without any discount so dey gave refund of my previous bill and generated separate bills for 19 packets twice.. So that discount was not provided.

I need to know the reason and store has to provide me for the inconveniences caused..