I have unaccepted the solution because I have found that this not only effects when I manually resend an invoice, but all invoices attached to reminders. As such it is no longer just a minor inconvenience, it is a MAJOR bug.  I can't use customized invoices because of this. Sending out invoices with WRONG amounts due is NEVER correct. 

THIS IS A FIX IMMEDIATELY LEVEL BUG.

This was initially posted 6 months ago. That a bug of this level hasn't been fixed in half a year is extremely disappointing.