Craft Commerce Negative Stock
Commerce does not have a back-ordering (negative stock) counter at the moment. To allow ordering of more items that you have on hand, you would need to either:

1) enable the 'unlimited stock' option on the variant which would not track the stock and allow orders of an unlimited number of variants.

2) Choose an arbitrarily high stock count to allow ordering above what you have on hand. For example, if you have 32 items on hand, make the stock value 2032, allowing for the stock to drop below 2000 would let you know the number of back-orders.

Either of these might not be ideal.

A third option could be using option 1 with a plugin that listens to the onOrderVariant event and updating your own stock counter within your own database tables.

We hope to have features like back-ordering in future versions of Craft Commerce, and you can vote for such features here: https://craftcms.uservoice.com/forums/295386-commerce/suggestions/7817436-pos-and-inventory-management

Hope that helps.