One of our clients has asked for the ability for its order entry personnel to submit orders with a negative order total. The application already allows negative line items, which works fine as long as the rest of the order makes a positive Grand Total.
It seems that Magento tries very hard to not have a negative order total. Looking at the collected totals shows that Magento will apply a gift card for the inverse amount. If I disable the gift card, then it applies Store Credit (even if credit does not exist). I suspect this is just a quirk (and possibly a red herring).
Can anyone help pinpoint where Magento forces 0 as its maximum Grand Total amount, and how we might go about fulfilling the request? I don't want to simply force-set the total, as I want Magento to calculate everything normally, just with the allowance of a negative Grand Total.
The spirit of the question is the technical feasibility, but once that is addressed, I'm interested in any 'gotchas' or thoughts that might dissuade the use of negative order totals from a non-technical standpoint.