Paytm Mobile Solutions — paytm cancelled my order without my consent

Address:	Gurgaon, Haryana, 122022
Website:	Paytm

I had ordered a moto Z2 play and while booking my order i was shown a cash back option of ₹5500 after applying some promo code and i opted for it . After 2 days they automatically cancelled my order without my consent and started the refund process. After i reached to the customer care they are saying that there was some technical issue with that promo code at that time, so we cant process your request and advised to place the order again with giving higher price. And now the offer is not there and i have to place the order again at higher price. How could they change that after placing a successful order. What do i do.