Hi Sam,
Makes sense as a statement of what you want.
But the current answer to your want is 'feature not supported.'
I suspect it is possible to build a feature like this into an Automator Service or an AppleScript. If that were done, calling the feature would be a simple matter of a keystroke combination or a menu choice.
Scripts acting on the content of a single cell are specific to the design of the document containing that cell, meaning that, as usual, the devil's in the details. A screen shoot of your invoice model, clearly indicating which cell (or cells) would contribute information to the file name would be useful to any of the talented scripters who frequent these waters. They'll also want to know the version number of Numbers you are using (Numbers menu > About Numbers), and the version number of OS X or macOS you are using.
Regards,
Barry