Bad service

I app**** for an iPhone 7 deal at Northgate mall mtn shop the one inside on 19 May 2020 and it was approved then I went to collect the phone they said they stock was over I have to wait for the stock after a week I decided to change the deal and take another deal since the stock was taking long to come then I went to mtn Northgate outside shop to assist me cancel the iPhone deal and apply another deal which was Huawei P30lite on 25 May 2020 they said my application was declined and I didn't understand the verting team what's the problem coz I was just changing the deal I already had been approved before,,,, um rlly angry about the service I had to taday. How is that possible to be be declined after been approved a week before????? Is that how mtn operates with customers???