After I thought about it, this worked. All entries dated today:
1. Create a CREDIT MEMO for the customer for the same items as in the invoice.
Add notes for a paper trail of why voiding an open invoice in a closed period.
2. Go to RECEIVE PAYMENTS for the customer and click "Apply Existing Credits?"
Leave amount as zero dollars and uncheck all "Outstanding Invoices/Statement Charges."
Check the invoice you want to void that matches the credit memo dollar amount you created in step 1. Save payment and the invoice will no longer show up A/R.
