There are four main steps involved in an exchange's typical flow of funds:
Alpha Exchange users (like Charlie) can trade credit-based balances on Alpha Exchange. Alpha Exchange should keep track of user balances on its new balance sheet as these trades are made. These trades are off-ledger and independent from the RCL, so the balance changes are not recorded there.
For more information about trading on the RCL, see Lifecycle of an Offer.
-Exchanges can adjust the balances between their operational and issuing accounts at any time. Each balance adjustment consumes a transaction fee, but does not otherwise affect the aggregate balance of all the accounts. The aggregate, on-ledger balance should always exceed the total balance available for trade on the exchange. (The excess should be sufficient to cover the RCL's transaction fees.)
The following table demonstrates balance adjustment of 80,000 XRP (via a payment on the RCL) between Alpha Exchange's issuing account and its operational account, where the issuing account was debited and the operational account was debited. If the payment were reversed (debit the operational account and credit the issuing account), the operational account balance would decrease. Balance adjustments like these allow an exchange to limit the risks associated with holding XRP in online operational accounts.