can't cancel the invoice

Hi

User is try to cancel the Invoice .But he is not able to
cancel .System throwing error is “Invoice
document can’t cancelled” but when I checked in HELP it is telling -this is
not possible as the document has not yet been posted. Message No M8490 .But for this Accounting
document generated. When I check work flow for this it is still in process. Then
why it happen .How to overcome this problem. Now user want to cancel the document.
Please suggest

Thank you

Dharma