You can use some sort of concurrency index on the ledger.

For example you read the ledger entries for Account Id X

Those ledger entries are numbered sequentially up to the current index Y

You calculate the current balance, approve the withdrawal, and try to insert the next ledger entry as Y + 1

If another operation has already inserted a Y + 1 your insert fails and you throw an error or try again, starting with the new ledger state and balance.

Basically optimistic concurrency but knowing that you are always appending to the ledger and that the index is always incrementing.