Question: I sold gold [jewellery] that I used to wear for some time but I did not pay Zakaah on it. I would like from you to make it clear to me what I should do about its Zakaah, given that I sold it for four thousand riyals.
Response: If you did not know that you had to pay Zakaah on it until after you had sold it, then there is nothing that you must do. However, if you knew that you had to pay Zakaah on it, then you should pay 2.5% per year. You should pay that for each previous year according to the value of gold in the marketplace. You must pay 2.5% in cash. However, if you did not know about that until the last year, then you should pay Zakaah just for the last year.
Shaykh Ibn Baaz