Hi Chee Chung,
My scenario only have downpayment and do not have invoice etc. Yes, i did test with "Credit Memo" as well but i also get the same error. Eventhough i enter as $1.
My users create the downpayment via Tcode: F-47 and make the payment via Tcode: F110. After that the downpayment will display at ME23N under status tab. Also, the material document number of the downpayment will display in the PO history tab in ME23N too.
Thanks.
Joo