As far as I can recognize, the account 57210 = Customer summary account (Posting profile in Sales ledger).
If the values in the screenshot represent a single transaction, then it is obvious, that it is a currency revaluation transaction. Because exchange adjustments impact only values in accounting currency.
If the values in the screenshot represent a balance of zero in Transaction currency, then it is based on a scenario like this:
customer invoice transaction = Debit 1000 USD - 800 GBP
customer payment transactions = Credit 1000 USD - 850 GBP
In that scenario no settlement between the invoice and the payment transaction was done.
If both transactions would have been settled, an additional transaction would have been generated: Debit Customer summary account / Credit Exchange adjustment (gain/loss) 0 USD - 50 GBP