I can't delete a sales order item which has canceled the billing document.

SO 22000001/20

-> Invoice for 2200001/20 2011/10/11 Completed

-> Accounting Document 44000001 2011/10/11 Cleared

->Cancel. INV F. 23000001/20 2011/10/13 Completed

->Accounting Document 28000001 2011/10/13 Cleared

I bill a sales order related item and then I found there is a error.

and I canceled this billing and want to delete this sales order item.

the system shows error message which shows "already has subsequent document"

but I have already canceled the billing. why can't I deleted the sales order item 20?