Hi,
System will generate entry in TBB1 would be Dr Bank 1000 (USD)
Cr FX clearing 1000 (USD)
The above entry will get converted into local currency as per the latest exchange rate i.e 9400 in your case and the difference amount 400 will be realized gain.
Here no question of clearing ZAR400 which was posted into Bank.
Thanks,
VNTR