If you want an overview of a specific client's invoices and credits, you can create an account statement directly from the client's contact details. First open the contact and then go to the "Finances" tab where you will see a summarized listing of all invoices and credits as well as a chart showing revenue amounts. Click "Show account" which will give you an opportunity to export the account statement.
You can make manual postings to the client account.
The overview of account statements created can be found under "Sales - Other - Account statements".