This would mean that the Consumer Protection Act applies, which is good news.

Section 15 of the Consumer Protection Act state the following:

15. Pre-authorisation of repair or maintenance services.—(1) This section applies only to a transaction or consumer agreement—
(a)with a price value above the threshold prescribed in terms of subsection (5); and
(b)if, in terms of that transaction or agreement, a service provider supplies a repair or maintenance service to, or supplies or installs any replacement parts or components in, any property belonging to or in the control of the consumer, and—
(i)the service provider has, or takes, possession of that property for the purpose contemplated in this paragraph; or
(ii)in any other case, the consumer requests an estimate before any services or goods are supplied.
(2) A service provider to whom this section applies, must not charge a consumer for the supply of any goods or services contemplated in subsection (1), unless—
(a)the supplier or service provider has given the consumer an estimate that satisfies the prescribed requirements, and the consumer has subsequently authorised the work; or
(b)the consumer, in writing, or by another recorded manner or form, has—
(i)declined the offer of an estimate, and authorised the work; or
(ii)pre-authorised any charges up to a specified maximum, and the amount charged does not exceed that maximum.
(3) A service provider to whom this section applies must not charge a consumer for preparing an estimate required in terms of subsection (2) (a), including—
(a)any cost of performing any diagnostic work, disassembly or re-assembly required in order to prepare an estimate; or
(b)any damage to or loss of material or parts in the course of preparing an estimate,
unless, before preparing the estimate the service provider has disclosed the price for preparing that estimate, and the consumer has approved it.
(4) If a supplier has provided an estimate for any service, or goods and services, the supplier may not charge the consumer a price for that service, or those goods and services, that exceeds the estimate, unless after providing the estimate—
(a)the service provider has informed the consumer of the additional estimated charges; and
(b)the consumer has authorised the work to continue.
(5) The Minister may, by notice in the Gazette, prescribe a monetary threshold for the purpose of subsection (1) (a).

From this it should be clear that the charge is illegal.

I suggest what you do now is to inform him of this section and inform him that he has until the end of the day to release the vehicle, failing which you will have no other option but to apply to the High Court for an order that he hand the vehicle over. You will then hold him responsible for all your legal costs as well as all the damages that you suffer due to his illegal retention of your vehicle.

If he still does not want to release it, unfortunately, you have no other option than to go and see an attorney who can do the necessary application for you.

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