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While filling a new line inside sales order, a warning showed up to warn the user about the quantity if it is going below zero (Stackable Product, Make to Stock), but this will not prevent the user from confirming the sale order and continue the process.
How can I prevent the user from confirming a sale order in this case?
You should have a look on the module
It allows to create rules on sales orders which blocks their confirmation when they are not fulfilled (a sale manager can still bypass the blocking).
There is actually a rule in the module which blocks the sale order when the virtual quantity of a product in a sale order line goes below zero. The rule is deactivated by default but you can reactivate it.
You can also create new rules if this one does not exactly fit your needs.