'Negative quantity' on sales order

[pricerc]

So I'm working on a solution for a new customer in a retail environment.

Their existing POS allows them to do a refund by 'selling' a negative quantity of an item. This is not something they do very often (but obviously often enough that they asked the question), typically for something like exchanging a can of spray paint for a different colour, where both items have the same value. They will scan one, change its quantity to -1, then scan the replacement. The order value is then typically nil and the customer goes away with a new receipt reflecting the exchange.

From the JIWA POS program (either the plugin or my standalone derivation thereof) I can change it to allow negative quantities, I then see two options for the underlying accounting:
1) create a credit note for the 'refund' and then the sales order, and apply the value of the credit to the sales order.
2) make the refunded item a non-stock item with a negative value on the sales order and then write it on to inventory using a transfer.

I figure that 1) is the 'better' accounting practice, but 2) would at least allow me to put both items onto a single invoice (which may be printed on a thermal receipt printer). I also suspect that 2) will be easier to code for.

Any recommendations or other suggestions?