An underlying design philosophy of Manager is not to get in the user’s way. There is no reason, from an accounting viewpoint, why an item cannot be sold for less than it cost. Suppose you want to clear out slow-moving merchandise? Suppose you want to attract customers by selling one item at a very attractive price point?
And many users buy on a just-in-time or as-ordered basis. They sell the inventory, receive payment, and only then order it from a supplier. This is very common with extremely expensive or customized products.
So Manager lets you do what you want. It is an accounting program, not a sales management program. So it records your actions rather than controlling them.