If you create an invoice and the customer only pays a part of it, then a part of it remains open. You can, therefore, complete the invoice with a certain amount as an outstanding amount. 

For example: the total amount of the invoice is 86.57, and 20 euro is paid in cash today, the remaining amount 66.57 remain open.
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​If you save an invoice as outstanding, the full amount of the invoice is stored directly with the turnover of that specific day. The turnover is therefore not divided over multiple dates. Payments (transactions) can be stored on different dates.
Payments are listed on the payment screen on the left side of the cash register. You can view or edit the details of the payments. When you click on the pencil icon, you will get a summary of the date and time when the payment was registered: 

Payments that are registered within the current cash register can be removed and modified. The bin-icon will be active. 

Those that fall within a closed cash up, for example payments from a week ago, can no longer be removed or modified. You will see that the bin icon is inactive, so you cannot delete those payment.
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Would you like to see the payments and payment dates for a completed invoice at a later time? Then you can look up the invoice via the register tab, in the invoice overview. Click on an invoice and click "edit" to see the payments and payment dates.