I set up a custom field for marketing purposes using drop down selection.  I made it required on all invoices & sales receipts.  So when an invoice is entered it can not be saved until they fill in this field. For example ,  they have to choose, web ad, trade show, referral, etc.  The problem is that if we do a second invoice for the same customer what ever was selected selected the first time for this customer is populated on the new invoice.  Is there a way not to save this info by customer?