Can you explain in detail (and probably with screenshot) what you are actually trying to do? 

Hi Adam ,

I try to offset customer debit invoice against a credit invoice with the same amount

And you really have an error “At least one Invoice Line must be entered before saving Invoice” in customer offset window? 

This error is raised normally when you are creating new invoice (so not in offset) or new difference item (which is not the case here as well).
If so, I suspect a bug to be reported to IFS