How do I update a field value on an invoice?

I am trying to create a user event script to update the value of a custom text field on the invoice when then invoice gets paid in full through the customer payment transaction.  The customer payment transaction lists the invoices to pay on the apply tab but I cannot find a record layout for the sublist similar to the items sublist, which I used for another script to update a custom field on the invoice.  Is there such a sublist or how can I update the custom text field on the invoice when it is paid in full through the