False Trade-in Promotion

MTN is running a false promotion promising customers discounts if they trade-in their old Samsung phones. I traded in my Samsung Note 20 for a Samsung S24 at Irene Mall in April to receive a R400 discount on my monthly payments. However, this discount has not been app****, and it is now 4th month. I have been paying the full amount each month without the promised discount. I have called the call centre, went to the store numerous times and they are unable to resolve the problem and they also cant give me my phone back.