Earlier quoted context omitted.
Slotting fees are real, but of course supermarkets pay for the product. How else would the manufacturer be paid? The fee is basically paying for the right to shelf space, but the supermarket still buys the product to fill the shelves. Also, not all supermarkets, including not all large chains, have such fees.
They're paid when the product clears. Many big chains like Walmart and Best Buy don't own a significant portion of inventory on shelves or in warehouses. The manufacturers own the inventory up until the products are paid for by the customer.
They simply might not have forked over the cash yet, which is very different--for a variety of reasons, the actual transfer of cash can take place weeks or even months later. (This is how invoicing works with most companies. If you are an independent contract, you know exactly how this works.)
Very few products in retail are sold on a consignment basis (meaning, when the product sells). Generally, only new products or slow-selling big-ticket items (like refrigerators) get sold on consignment (or similar) arrangements.