If I understand correctly the idea, the store's checkout system 1/ generates a key pair 2/ prints out the QR code of the privkey on the cash receipt and 3/sends out the change to the bitcoin address.
One slight problem some might have with that though is that the customer cannot redeem the code instantly -- as the transaction by the merchant to load the funds to the address on the QR code from the receipt doesn't confirm immediately. The key from the QR code could be imported immediately, just that it couldn't be redeemed / swept immediately. This probably isn't a major problem but it detracts from the benefits of the concept slightly.
The customer can easily redeem the privkey on strongcoin.com or equivalent at a later time when its convenient for her.
A problem
might will occur where there is difficulty in redeeming the code. When the redemption doesn't occur immediately (e.g., while still in the store) then some of that "difficulty" will be the result of attempted fraud (e.g., claimed the code then makes a complaint that the code couldn't be redeemed.) and some will be for other reasons (customer couldn't figure it out, technical problems, etc.).
The risk of fraud will probably kill this idea before it even gets off the launch pad though. The employer cannot prove the customer is in the wrong, so the end result is either the merchant ends up denying refunds to innocent customers who were truly unable to obtain their change (e.g., when the merchant itself runs an insecure system) or the merchant gives refunds for most claims and as a result the amount of fraudulent claims (and thus the cost of offering this) skyrockets.
So maybe requiring the customer to provide the public bitcoin address (obtained with a card swipe perhaps) is the only workable method for this. There's no liability on the merchant's end -- the receipt provided shows the bitcoin address that was scanned and includes as well transaction info for the change payment that can be verified against the block chain.