It's a great idea - but then there'd would be a risk: customers would have to trust the business 100%.
If the restaurant immediately made a tx to the change address, providing the txid on the receipt, would not that be sufficient. The customer would also know the restaurant, so if there were any troubbles he'd simply go back there and hassle them.
If the customer were paranoid, he could simply issue a transaction to himself for the change and then hand over the privkey to the restaurant which would contain the exact amount. The restaurant should then of course immediately transfer the balance to a company address.
I don't see why a reputable business should have an interest in not giving the customer change back.
The whole point would be to avoid problems with tx'es not going through immediately. You can't sit and wait for too long after you've finished your meal, most people are interested in leaving.
So payment could be just you presenting the 'value-bill', the waiter scanning it and then sending you the change and giving you a receipt. That way, even if there's congestion or other issues sending the transaction, it doesn't matter, because you've paid already, and the waiter could check against the blockchain that the privkey and its corresponding address holds the required value.
I guess the customer could always try to double spend once he leaves the restaurant, but what would be the point in that, and is that a legitimate concern?
This could also be a backup feature of an existing POS-system, kind of like the reserve aka offline transaction done with a VISA card or debit card when there's no connection with the bank and you just swipe your card and sign.
But the problem is as shown in the link I presented in the first post that if you already have issued a tx, it's quite likely to go through after some time, and if you then use the reserve solution, then you'll pay twice.
But I think that it would not be too difficult to guestimate how much you need for a restaurant visit, then loading what you need on a 'value-bill', or on your smartphone - whatever you think is better - before you go to the restaurant. Having it printed on paper would also ensure that in the event your phone runs out of battery, wifi or roaming is not available or your phone is stolen or wrecked, you can still pay with bitcoins. If your phone is inaccesible and you want to pay with btc, there's no way to do that unless you have paper backup aka a 'value-bill'. I guess bitcoin enthusiasts most likely want to walk around without needing a VISA card. I guess most people have both these days, but if it can be paid with bitcoins that must be the desired option.
And of course, this must be a rather simple procedure from the restaurants point of view, it should be so easy that the waiter can learn it in 30 minutes. Everything should be automatic really, there should just be a few manual steps for the waiter, and he could either use a handheld device, a smartphone, or the customer could be asked to go to the cash register where they have a setup for payment.
