Is it permissible to purchase an ounce of gold with a partial upfront payment, and receive it later at the same agreed-upon price, knowing that the shop does not deliver the gold immediately and sells it by order?

It is not permissible to enter into a contract for the purchase of gold and defer its possession, because it is one of the usurious commodities for which immediate possession is a condition at the time of the contract. However, a non-binding promise to buy is permissible, followed by the sale after the seller takes possession of the gold, and possession occurs upon payment of the price.