Backend checkout system?

So my client has an online shop and a physical shop.

He wants to be able to make manual sales on the computer in his shop when ever someone physically buys something.

This is so that he can use the Manage Orders screen for in shop orders and keep log of all sales online and offline in the one place. And will reduce the stock counter for the online shop. As he has had issues where he has over sold an item because someone picked one up in the shop and then someone else ordered the same thing online.

Basically I would have to recreate the checkout system in the admin area, then process it without a response from a payment gateway. Maybe mark it as Cash Payment etc.

Is it possible?