How can I manually bill an outgoing invoice?

Steps

1. Search for / filter for your outgoing invoice (e.g. “Status” == “finalized”)
2. Double-click the line featuring your invoice
3. At the end of the new page, click “Payment”
4. In “Payments”, you can now either …
   1. approve payment of the entire sum by simply checking the check-box “Set invoice to paid”
   2. approve partial payment of the overall sum by changing “Amount” to the currently received amount (HINT: note that “Open amount” will now show the difference between what has been and still needs to be paid)
      => HINT: If you approve a partial payment for this invoice, then you will see the open sums the next time you view the “Payment” option for this invoice)
5. Click “Ok”
   => The full / partial payment has been booked
   HINT: If you wish to book a full payment for an invoice while at the same time granting a discount, simply enter the discounted sum (i.e. invoice amount minus discount) in “Amount” and check the “Set invoice to paid” check box)