Billing Overcharge in breach of Contractual Agreement

I am disappointed with Vodacom in not fully resolving my FTTH Billing query. 1. During Oct/Nov 2024 I was going to cancel my FTTH after the initial 24 month contract expired. The Retention department convinced me to stay by offering a discounted package at R599 pm on a 50/25 mbps, which I agreed to in good faith. 2. However, even though my contract stated R599, Vodacom proceeded to bill me R699 per month. I didn't notice this at first, but when I did notice I called them to query, upon which the billing department confirmed that it is not aligned to my contractual agreement. 3. Instead of reimbursing me the full amount I was overcharged every month, Vodacom gave me a Credit note of R300, indicating that the case is now closed. 4. To add Insult to Injury, they still proceeded to bill me R699 instead of R599 on the December Billing statement. 5. I view this as misleading or even de*****ing me as the customer by not honouring the contractual agreement that we entered into, which is breach of contract... What right of recourse do I have before approaching the Ombudsman?