You describe modifying a portal row, but you do not tell us which table occurrence was selected for the portal nor which was selected for the portal's layout. We need to know that in order to help you.
In general terms, you would want a separate line item for each job's invoice, not the same line item linked to multiple invoices.
The "more than one" is a good time to consider "Many-to-Many" via a join table.
Jobs -< jobs_invItems >- InvoiceItems
Think about what you have and what you want and where the data needs to reside.