order confirmed

I made an online purchase I got the email confirming that my order was placed with reference P ********** 71 on the 24/01/2018. the is an item which was cancelled by TFG with code ********** which I already paid for they cancelled it without informing me that it was cancelled. when I call them to find out what was the reason for the cancellation they told me that they are out of stock. however when I confirmed the transaction the item was available now they tell me that I have to purchase it at the shop which I will no longer get the discount/promotion of that 50% ill have to pay extra from what I confirmed the transaction