"It has since been upgraded to FM7" does that mean that this is the version of Filemaker that you are using? This is a very old version.
One method is define a global field and set up this relationship:
Globals::GlobalField = Invoices::Jobnumber
You can then set GlobalField to the value on a layout based on Globals.
Not IsEmpty ( Invoices::JobNumber )
will be true only if the jobnumber already exists so this is a way to look for an existing value before you create a new record with it.