Is exchanging old gold for new gold permissible or forbidden, and if it is forbidden, what should be done with the gold that has been exchanged?

It is permissible to exchange old gold for new gold if they are equal in weight, present together, and the possession of neither is delayed, according to the Prophet's (peace be upon him) saying: "Do not sell gold for gold unless it is like for like." If the exchange involves an increase in one of the gold items or a delay in the possession of one of them, then the transaction is void. What was given by each party must be returned if possible. If it is not possible to annul the transaction because the other party is absent, then one must repent, seek forgiveness, and increase good deeds, because usury (riba) is one of the major sins.