Edit Customer Invoice

[Jim M. <jim...@wo...>]

It appears a user (any user) can edit an existing invoice even after it's
created/posted.
Using the demo: http://business.dynodns.net/cgi-bin/sql-ledger/login.pl
I was even able to modify existing invoice numbers from Receipts and
create/post multiple invoices with the same Invoice value (1007 in our case
using Customer: ABC Widget Company).
We removed the 1007 invoices (using Delete Invoice) so it shouldn't impact
any other testing/demo. This appears to be a problem.
The procesing under Post Invoice button should check for the existence of
the Invoice of the same value prior to creating another. Otherwise two
invoices with the same value can be generated, even though the id is
different. Not sure of the impact down the line in other tables.