You have 2 options:
From your client list, do an advanced search enabling the columns that you wish (such as the ‘Total Invoiced’, ‘Total Received’ and ‘Prepayments’ options).
Running the ‘Debtors (Clients) Ageing Report’ will list all clients with a balance, and also show their prepayments at the side. While this doesn’t show you the actual balance, you can export the data into Excel and perform calculations on it as you wish. This can be found by going to Reports >> All Reports >> Debtor (Clients) Ageing Report
Or, if you were after a complete list of outstanding invoices, the Historical Invoices report may be of more benefit to you. Again, pre-payments aren’t shown in this report, but you can export them to a csv file. This is also found in the ‘All Reports’ section.
I hope that helps?