[yobbo]

Distinguishing between sales and payments is required, but any confusion is basically impossible in online systems such as this schema would be used in, in real examples.

Typical accounting GUIs present transactions and ledgers such that it becomes intuitive for business users to enter sales/invoices. Entering the above transaction in those GUIs might cause confusion.

The schema here is used by an online system recording business events as a type of log of user interactions. It lives in a sql database, and thus needs to rely on row constraints to guarantee double-entry. Transactions are never entered manually or randomly.

The effect of split transactions can be achieved by using an intermediate account. Those accounts have balances which are not guaranteed to be zero, but that is unavoidable and sometimes OK and intended.