A business logic vulnerability in SourceCodester Pharmacy Product Management System 1.0 allows attackers to manipulate sales requests to purchase quantities exceeding available stock. The vulnerability exists in the add-sales.php file where the application fails to verify if the requested sales quantity (txtqty) exceeds available inventory levels. This oversight enables attackers to exploit the system by submitting purchase requests for significantly higher quantities than what is actually in stock, potentially leading to inventory discrepancies and business logic bypass.