It is probably uglier than that.  Did the person posting the invoice get a record lock and have to quit posting hte invoice midway through?  If that is the case and some lines posted and some didn't, you need to make inventory adjustments to put the parts back into stock that DID post, delete the revenue transactions that are causing the out of balance, then release and process the invoice again from the beginning.
 
Then make sure you are up to date.  The August IS Tech update introduced a new temporary holding file that will prevent half-posting of invoices due to inventory record locks.

This is great news!  We don't have this happen very often, but I have seen it and is IS really ugly.