Make it do not bill time. By definition do not bill time won't appear on the invoice.

My boss uses "Do Not Bill" for internal troubleshooting, so that is not a good option, especially when it comes to reports.  We talked about marking all of those slips on "hold", but that would mean going back later and creating a zero invoice just to bill those out (and would not connect them to the invoice for which the work was performed.).

Any other suggestions?

You can generate a bill and exclude slips marked "No Charge" in the filters.  But then the slips will still not be associated with the invoice they are attached to.  

Maybe you want to print a bill and exclude the "No Charge" time ... use this to send to the client (without approving) and then re-generate the bill and include the time for approval purposes.  You would need to keep a clean PDF of the bill that was sent, but it would hide those slips from the bill and the "approved" copy will associate the slips with the invoice.  Is that too confusing?

No, I totally understand.  I have done that before.  This particular client is asking for a separate invoice for each reference code (billing code) which totals 113 invoices just for this past month (with more to come in the future).  I was hoping to find a way around double work. lol.  Even creating a manual invoice would still be double work.  

Still hopeful that someone MIGHT know if there is a solution, perhaps in the bill layout or something.

I really do appreciate the help and suggestions!!

Only solution that we came up with on our own.  We created a task for "No Charge" for those specific slips, then made them "Do Not Bill".

This way they do not show up on the invoice, however they are "marked" for our reporting/auditing purposes.