can't edit memo field on posted invoice transaction

[damon]

I want to edit the memo field on a transaction created by the posting of
an employee voucher (which is like an invoice?).  I want to enter the
receipt for the expense in the memo field so that I can easily find it
later(like for an audit).  These transactions are read only because the
are tied to an invoice entry.  This makes sense, but why can't the memo
field be edited?

The transaction doesn't exist until I post the invoice / voucher, and
after that it is read only.

gnucash version = 2.4.13


-- 
damon