Products (or variants) have attribute negativeStockAllowed, which allows to sell a product (variant) even if it is not on stock. If global eshop settings are “not allowed”, this attribute can override the global settings. It is possible to allow a product (variant) in case the global setting is “disallow” but not vice versa (if global setting is “negative allowed”, you cannot prohibit it for one product/variant).
Item data.variants[].negativeStockAllowed can be provided for all Product insertion, Product update and Product by code update endpoints. Valid values are true or false.