If you tab to the Number field on invoice, you don't have to delete the existing number first.  Just start typing your 'manual' number and the pre-filled one will disappear.  When you tab to that number field (as opposed to placing your cursor in it), the entire number should be highlighted in green, which indicates when you start typing the other number disappears.  If you are using the mouse and the cursor is placed at the beginning of the number field, then you can type your new number but will have to delete the old one.