Is it permissible to exchange old gold for new gold with an additional sum of money, and does the statement that gold has become a commodity bought and sold like other commodities justify selling it on credit or exchanging it for an additional sum of money?

Equality is obligatory when selling gold for gold, and any excess (riba al-fadl) is forbidden. The good and the bad, raw gold and minted gold, and intact and broken gold are all the same in terms of the permissibility of selling them with equality and the prohibition of selling them with any excess. It is not permissible to exchange old gold for new gold with an additional sum of money. Rather, it is permissible to sell the old gold first, then add contemporary currency to complete the price of the new gold. Contemporary currencies take the same ruling as gold and silver regarding the prohibition of riba al-fadl (excess interest) and riba al-nasi'ah (interest due to delay).