What is the ruling on exchanging new gold for old, with the payment of the difference in value?

It is not permissible to exchange gold for gold unless their weights are equal, because they are among the usurious commodities in which superiority is not allowed. This is known as riba al-fadl (usury of surplus), and it falls under the severe warning in the verses of the Holy Quran and the Prophetic hadiths that describe the one who consumes usury, the one who gives it, the one who writes it, and its two witnesses as being equal, and that usury is among the greatest sins by which Allah is disobeyed, and it is considered a war from Allah and His Messenger. The best solution is to sell the old gold and receive its price, then buy the new gold with that price.