The workflow for reversing a job sales invoice is that you need to create a new job planning line with a negative qty. To impose exact cost reversing, the Type field (e.g. Resource, G/L or Item), No. field, unit price and unit cost in the new job planning line MUST BE THE SAME with previous job planning line where you posted the sales invoice from.
Once you have the new job planning line(s), use the Create Sales Credit Memo action from the ribbon of the Job Planning Lines page to transfer the job planning line to a new sales cr/adj note document. Then. post as usual.