How to add the ability to sell products on order in x-cart?

Question

CMS

Good afternoon, I
installed x-cart for myself
and I want to add such an opportunity that the balance of the goods is taken into account
and if it is equal to 0, then it was possible to click buy, but at the same time it was displayed, under the order or a similar inscription. Well, it turns out that the balance left in the minus.
Has anyone implemented this, maybe on another cms?