I recently took over bookkeeping for a client.  Upon reconciliation, I noticed that there was a duplicate billable expense in the register.  I deleted the duplicate (or what I thought) and it accidently deleted the one that is linked to a paid invoice versus the other one.  When I go to try to add the ones that were kept to the invoice, they don't show as available to add.  But they are still in the register and marked as Billable.  Is there a step that I am missing?  Or is it not possible to add a billable expense to an already paid invoice?  I need to add it so that they payment amount matches the invoice amount.  Any suggestions?