Some practical cases where you really need such a system:
A customer pays one product with two different sources of payment (gift card + visa, or one half with cash and the other with card)
You want to verify if the balance in the cash register, or on the credit card account are correct.
You already received money from a customer, but it will take some time until you can create the invoice (for example you want to print the serial number on it, and don’t know it yet), but for legal reasons you are not allowed to change an invoice later. So you have to record a payment without having an invoice yet.
A customer pays one product with two different sources of payment (gift card + visa, or one half with cash and the other with card)
You want to verify if the balance in the cash register, or on the credit card account are correct.
You already received money from a customer, but it will take some time until you can create the invoice (for example you want to print the serial number on it, and don’t know it yet), but for legal reasons you are not allowed to change an invoice later. So you have to record a payment without having an invoice yet.