Hi,
the simplest way:
1. Unapply the credit memo (if it's applied against the original invoice)
2. Create a new invoice, use "copy document" and copy from the erroneous credit memo, including head, don't recalculate lines.
The new invoice should directly apply to the credit memo you have copied from (default behaiour). Upon posting the invoice, the customer balance is cleared. There is no other way in NAV. Document-bound postings cannot be reversed, this is by design.
with best regards
Jens