What is the ruling on giving money to a goldsmith to buy gold when its price is low and sell it when its price is high, and what should I do with previous profits if there is any doubt about them?

For the buying and selling of gold with money, immediate possession in the same session is a condition, based on the Prophet's (PBUH) saying: "Gold for gold... hand to hand." Monetary currencies take the same ruling as gold and silver. If you give the jeweler the money in advance, it becomes a debt upon him, and the condition of "hand to hand" is negated. The permissible alternative is to keep the money and monitor gold prices, and when the price drops, you buy gold and keep it, then sell it when its price rises. As for the profits gained from previous transactions before knowing of the prohibition, there is no sin in them, for Allah says: "So whoever receives an admonition from his Lord and desists may have what is past."