Reverse Credit Memo? Nav 2009

Hi, we have Nav 2009 R2, we have a credit memo that is built up of lines of Items and resources, the items have Zero cost & are required to be on the credit memo as they are being returned to stock (customers own stock) but the resources have costs, but should have not been in the credit memo, this is a user error.

Both have been posted back & hence the problem s it is trying to credit the customer the value of the resources - which is incorrect.

So, can i reverse either the Resource lines on the Credit memo?

Or can i reverse the whole Credit Memo & create a new/correct one,

Thakns

Ian