You do have the right in terms of the National Credit Act to voluntarily surrender any goods bought on credit. Section 127 of the National Credit Act talks about Surrender of Goods.
The Act states that you can give written notice to the credit provider that you want to return the goods. Within 5 business days you must voluntarily surrender the goods to the credit provider’s place of business or any other place as agreed upon with the credit provider. Within 10 business days after receiving notice the credit provider must give you written notice setting out the estimated value of the goods. If you are not happy with the value as put forward by the credit provider you can withdraw the notice that you have sent to the credit provider and resume possession of the goods…. unless you are in default under the credit agreement.
The credit provider will typically sell the vehicle at an auction. There are no guarantees that they will be able to sell it for what you still owe on it. If they sell it for less you will still be liable for the shortfall. If they sell it for more than what you owe they must pay over to you the difference.
I would suggest that you first talk to them to see if they can extend your term which will reduce your monthly instalment. With an extended term there is usually also a better interest rate. Do not forget to ask them about this.
Alternatively, you can sell the vehicle privately. you will most definitely get more for the vehicle if you sell it privately.