Just wanting to triple check my answers.

If a theoretical company wanted to defer the current periods advertising (provided by an external agency) to delay the billing of December advertisements until the next month in January, this would violate accrual accounting principles.

Since the work has already commenced, it must be recorded as such, resulting in either a decrease in cash (assets) or an increase to accounts payable (liabilities).

The other option is for staff to alter the invoice received. This would be unlawful since it is a legal document.