What is the X-Cart version? Is it X-Cart 5, or X-Cart 4? Please update your forum signature and read the forum rules.
In both versions the store administrator can reduce the stock level by editing a product.
Another way is updating stock levels from a CSV file. For X-Cart 5 there is a module:
http://kb.x-cart.com/en/modules/update_inventory.html
Plus there is a solution that automates the whole logic:
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https://market.x-cart.com/addons/point-of-sale.html
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https://market.x-cart.com/addons/POS-system-for-X-Cart-4.html