To view a client's paid or unpaid invoice, search for the client in the Client Search to the right, then click on their name in the Search Results:
From here, click the Actions menu to the right, then click Invoices from the drop-down menu:
Their Transaction History will pop-up, and you can click View next to the relevant invoice:
If the invoice is owing, then you'll have the option to Pay Invoice, as well as the option to Email the invoice to the client, or Print.
Clicking Print will also allow you to save the invoice as a PDF:
If the invoice is paid you'll also have the option to Email or Print, but you'll also have the option to Refund the transaction (this will take you over to POS with the items so you can process the refund):