@PhilMcIntosh - one of the great things about applying receipts from a national account is not needing to know which account the invoice belongs to as the national accounts receipt application handles this. However, if it's really important to your client, you can use this query and set it up for them using MS_Query.
select idcust
from arobl
where IDINVC = 'IN1832'
Obviously they would enter the correct invoice number for the IDINVC field. If they want more information, we have actually create a small utility whereby a client can enter the invoice number and the customer name, number, payment status and a few other informational items are returned into a small screen.
Lloyd