Program AMBS6XFR for creating invoices

I'm puzzled and wondering if anyone can explain this?

Program AMBS6XFR is the one that creates an invoice and loads file
MBF9REP with invoice header data.    When the program does the WRITE to
the file it is not followed by an FEOD nor does the file have its
'Records To Force A Write' attribute set to 1.

Therefore the record should still be in the program's file buffer.  But
just about 10 lines of code later the user exit UMBINV1R is called.

I would think that the user exit program would not be able to see this
new record and therefore could not update it.

What am I missing here?   By the way, this is one of those 300 page
Synon spaghetti code monsters.

Also the program appears to update the OPENAR file after the user exit
is called.  I could be wrong on that but that's what it looks like to
me.
If I want to change the Aging Date using UMBINV1R it won't find the
OPENAR record either.

Greg