It is not permissible to exchange old gold for new gold and pay the difference in value
Question

                Is it permissible to exchange old gold for new gold, and pay the difference in value to the goldsmith?.
Praise be to Allah.It is not permissible to 
    exchange worn-out gold for good gold and pay the difference. This is haraam 
    and is not permissible. That is indicated by the reports proven in 
    al-Saheehayn and elsewhere in the story of Bilaal (may Allaah be pleased 
    with him), who brought some good dates to the Prophet  (peace and 
    blessings of Allaah be upon him) and he said to him: “Where did these come 
    from?” Bilaal said: “We had some bad dates and I paid two saa’s of them for 
    a saa’ so that the Prophet  (peace and blessings of Allaah be upon him) 
    might have some of it.” The Messenger of Allaah  (peace and blessings of 
    Allaah be upon him) said: “Oh! Do not do that, (it is) the essence of riba, 
    the essence of riba.” Narrated by al-Bukhaari, 3212. 
The Messenger  (peace 
    and blessings of Allaah be upon him) stated that giving more when equal 
    amounts should be exchanged because of the difference in quality is the 
    essence of riba, and it is not permissible to do that. But the Messenger of 
    Allaah  (peace and blessings of Allaah be upon him), as was his wont, 
    pointed out the permissible way. The Prophet  (peace and blessings of 
    Allaah be upon him) told him to sell the bad dates for money (dirhams) and 
    then buy the good dates with the money. 
Based on this we say: If 
    a woman has some worn-out gold or gold that people no longer wear, she can sell it in the market and take the money, then buy good gold of her choosing. This is the way taught by our Prophet  (peace and blessings of 
    Allaah be upon him),