"You cannot do this; the templates are matched to their Transaction Type."

I'm pretty sure that you can.  Let's say that a customer walks in the door, selects a few items and asks you to ring it up.  You create an invoice and print it.  He looks at it and says, "wait a second.  I'm not sure about this.  I need to ask my wife."  You certainly can mark the invoice as pending (at least you can on my version) and you can title an invoice anything you want.  You can call it Bill of Sale, Receipt, or Vanilla Malted.  

If the OP wants to hand a piece of paper with the word Estimate at the top, he can do that, and marking it Pending reverses the sale.  He can mark it Final to recreate the sale.