It can only be lawful if it is cash bargain and the gold is physically transferred to the buyer at the same time either by hand or via bank account from which the buyer can get it as real gold. Otherwise it would be considered as usury. Because our prophet stipulated that the gold transaction is to be cash bargain and the gold is to be received/shipped right away.
“Gold for gold, silver for silver, wheat for wheat, barley for barley, dates for dates and salt for salt is to be paid same quantity for same quantity, equal for equal and payment being made on the spot. If the kinds (qualities) differ, sell it as you wish as long as the payment is made on the spot.” (Muslim, Musakat, 81 )
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