Is it permissible to sell goods by installment with a 30% increase over their cash price, with the possibility for the buyer to cancel the agreement before receiving the goods, even if the seller has already purchased them?

The permissible way in this transaction is for the agreement between your friend and the customer to be merely a promise to sell before your friend acquires the commodity; otherwise, it would fall under selling what one does not own, which is prohibited. As for increasing the price of a commodity sold in installments, it is permissible under certain conditions.