Not sure why folks are saying he's not due to pay something.
Problem is , someone is going to pay as SARS won't care. The client is not going to pay as he will hold the company liable, while he is liable to SARS, its thier mistake and has every right to ask for those funds or a reduction. Now whether he (op) personally is liable for the same amount depends very much on company policy, you need to read your contract.
1. If they have liability insurance, you may be asked to pay the excess ( thats quite often the norm ).
2. Likely to face a disciplinary but if you are training, it will be more against your manager than you.
3. This is the real world and actions have consequences BUT you owned up to your mistake and reported it, that goes a very long way in mitigation for a trainee, we were all there once. Depends on how the company handles it.
Personally I think your manager/mentor is in more sh** than you.