Is it permissible for a customer to give money to a grocer for the grocer to buy Pepsi for him from the company, and is this considered a Salam transaction or selling what one does not possess?

If the customer gives money to the grocer to buy goods for him and then sell them to him, this is valid as a "Salam" (forward sale) if the conditions for Salam are met, the most important of which is the full payment of the capital to the seller at the time of the contract. If the full price is not paid, it is only valid for the portion that was received, and according to the Malikis, it is not valid for the entire transaction.

If the customer appoints the grocer to buy for him, this is a permissible agency, and it is a condition for an agency with a fee that the fee be known. As for selling what one does not own, it is conceivable if a sales contract other than Salam occurs for a specific quantity that the seller does not own at the time of the contract.