No balance is stored as a number someone can edit. A balance is computed by adding up the lines that name that account in the append-only journal, in integer micro-units. A correction is a new entry, never a change to an old one, so the history of an account can always be replayed.
Money entering the platform comes from an external account, today only the demo credit account, so the sum over every account is always zero. That makes the whole ledger checkable: the running gateway audits its own memory against the journal, and the figures must match exactly or the process stops.
Alongside the balance, an account carries holds: amounts reserved for calls in flight, which are not yet spent and not yet anyone else's.
Example: bob signs up with 5.00 EUR of demo credit, spends 0.035 EUR on one call, and has a balance of 4.965 EUR, made of two journal lines and nothing else.