Hi,
If your bank reconciliation is up, then the bank sub-accounts are cleared once you upload the bank statement for each account (BUY & SELL), so in this case, the posting is transfered to your main Cash In Bank accounts. In your BUY transaction, your GL for bank is debited since there's a cash inflow while it is credited in your SELL transaction since you've sold ZAR currency in order to purchase USD.
I noticed that your Forex clearing account has a difference 'coz the exchange rate used for Buy and Sell are different, thus, this account is not fully cleared. To clear, a realized gain/loss should be recorded. Try to post the difference via TPM18.
Regards,
Chaikaru