This is the default situation for anyone buying from a wholesaler and selling retail, and doing it with two separate product records — one "case" and one "single" — is how stock figures quietly drift apart over a few months.
With a conversion defined, a case broken open on the shop floor does not need a manual adjustment. Cost per piece follows from cost per case, which also keeps margin reporting honest at the level you actually sell.
Where it gets awkward is a product whose case size changes between suppliers or shipments. That is a real situation and worth raising with us with your specific case rather than assuming either that it is impossible or that it is automatic.