Manually Assign Invoice Number

In this post: http://forum.opencart.com/viewtopic.php?f=20&t=48348 the following advice was given:

harald wrote: I have managed to solve this myself.
I simply added a input field and button in the order_info.tpl and changed the ajax request as send by the #invoice-create click event to add the value of the input field. Accordingly I have made changes in both the controller (added the input field value to the the call for the model method) and the model (actually saving the inputfield as invoice no.)

My questions are:
What exactly did you add to the order.tpl and controller? Which files exactly?

From this post: http://forum.opencart.com/viewtopic.php?t=6880 I learned how to change the order number, but I want the order number and the invoice number to match?

Can anyone help?