Cancel the credit memo

Dear Guys ,

the user has posted the credit memo accidentally instead of Posting the invoice in MIRO txn for the PO . Coudl you Please tell me how to cancel this credit memo . We are using MR8M txn , with the reversal reason as "02" the user is getting a error message as " Quantity invoiced greater then GR quantity" . But if i use reversal reason as " 01" then the allowed posting periods 06/2011 12/2010 are allowed for the compnay code is displaying .

Help me regadring this issue .