Yep, I said I was naive. The answer was obvious to the AP person after we talked it through - “why don’t you do an invoice for that one, since the debit memo won’t let you do a negative?” Right she was. That was a team effort!
Landmine: whereas the debit memo pops in the amount by itself, the invoice (for that negative advance) does not, so we had to type it in by hand in the Ext Amount for the line (NOT the Less Adv Pay amount)