What is the ruling on selling old gold for new gold?

There is no harm for your wife in selling her gold in the gold market, and then purchasing with the money whatever gold she desires. However, it is not permissible for her to sell old gold by exchanging it for other gold while paying the difference, because this is the sale of one usurious commodity for another of the same kind with a difference in amount. The Prophet, peace and blessings be upon him, forbade the sale of gold for gold unless it is like for like. The lawful solution is to sell the gold for dirhams (currency) and then purchase the desired gold with those dirhams.