What is the best solution for investing in the price of gold on a particular day, given that the shop does not have gold available and orders it from a supplier in the capital weekly, and is it permissible to authorize the shop owner to purchase gold despite knowing that he deals with bank transfers?

Purchasing gold with gold, silver, or banknotes (Riyal, Dollar, etc.) involves riba an-nasi'ah (usury of delay). It is a condition that the exchange takes place on the spot, with immediate possession by both parties before they part ways, based on the hadith of Ubadah ibn al-Samit: "Gold for gold, silver for silver, like for like, equal for equal, hand to hand. If these types differ, then sell as you wish, as long as it is hand to hand." It is not permissible to delay payment or to delay the receipt of the gold. Giving money to the shop owner to act as an agent for the purchase without taking possession of the gold does not absolve one from violating the lawful contract. The solution is to appoint another person in the location where the gold is available to purchase it for you, pay the price, and take possession of the gold in the same sitting, and then send it to you later.