In generate cost based invoices you have an option to bill for items based on a specific cutoff date: either the WIP Date or Accounting date.  If the tiem(s) you want to include have the same WIP and/or accounting date as items you do not want to bill for on this invoice, you can go into Change WIP and change the status on the costs that you want to exclude from the bill to HOLD. After you have biled the costs you want to bill go back into Change WIP to change the status back to Bill

ok that makes sense, I will try it on our next billing cycle. Thank you for you help!