Can't change Billing Terms on Printable Invoices

[Mark Baustian]

Using 2.2.9 on debian sysem.

I set my customers up with billing terms as 'Net 30' (set up using the Billing 
Terms Editor) and when I create an invoice, in addition to my expectation 
that the billing terms should import with the customer, I set the terms 
to 'Net 30' on the invoice as well. Yet when I try to print the invoice it 
shows the terms as Balance Due. If I use a template that includes a due date 
(which I generally do not) the due date is correctly calculated based on the 
Billing Terms I select. When I close the invoice (whether posted or not) and 
reopen it the billing terms on the invoice have reverted to 'None' although 
the terms associated with the customer remain as 'Net 30'. I've scanned 
through the fancy report scm and find where the Terms are specified in the 
report but I do not find where the  value is assigned to the terms.

I have no experience writing scripts but have programmed enough to read them 
and have a vague idea what they are doing. I'm not afraid to edit them but 
explicit instructions would be helpful.

thanks in advance, Mark