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

                Is it permissible to exchange old gold that I have for new gold, and pay the goldsmith the difference in price?.
Praise be to Allah.
    It is not permissible to exchange low-quality gold for good gold and pay the difference. This is haraam and is not permissible, and the evidence for that is the report narrated in al-Saheehayn and elsewhere in the story of Bilaal (may Allaah be pleased with him) who came to the Prophet  (peace and blessings of Allaah be upon him) with some good dates and he said: “Where did you get these from?’ Bilaal said: I had some bad dates and I sold two saa’s of them for one saa’ so that the Prophet  (peace and blessings of Allaah be upon him) might eat from it. The Messenger of Allaah  (peace and blessings of Allaah be upon him) said: “Oh, do not do this, it is the essence of riba, the essence of riba.” So Messenger  (peace and blessings of Allaah be upon him) explained that adding more to things that should be regarded as equal because of the difference in quality is the essence of riba, and it is not permissible to do that. But as he usually did, the Messenger of Allaah  (peace and blessings of Allaah be upon him) showed him a permissible way. The Prophet  (peace and blessings of Allaah be upon him) told him to sell the bad dates for dirhams, then use the dirhams to buy good dates. Based on this, we say that if a woman has low quality gold or gold that people no longer wear, she can sell it in the market, then take the money and buy good gold. She should do it in this way that was taught to us by our Prophet  (peace and blessings of Allaah be upon him).