phone contract double charge

My wife took out a contract for a samsung s5 mini...as it was on promotion. Filled in all the forms got the approval and everything,but when she went to pick up the phone they did not have any stock and they will only be getting in stock in a week or so. Then my wife asked the sales assistant if she could rather get a phone that was in stock as she needed a phone that week. He then said they have the s5 in stock. She then agreed to rather take that one then. She filled in the forms again and got approval...got the phone and my wife was happy... now 3 months later we get a bill for R4000 for a penalty fee for the s5mini which she never took. <br> My wife and I are very negative as we will be paying for a phone that we never received and that the sales assistant assured us that by getting the S5 will not be an issue. This is not acceptable. Please we need clarity on this. I will not pay R4000 for misinformation.