Purchase order - negative quantities

Hi,

congrats with this marvellous product. I work voluntarily at a non-profit distribution company of beverages. Our product is bottled in a returnable bottle with a deposit.
In order to be able to process this it would be nice to be able enter negative quantities in a purchase and sales order.

I do not think this is difficult to implement  - most likely it is a matter of turning off some field checks. Maybe it would be nice to make it a parameter on item, item category or company level.

Thanks.

Mork - Belgium