What is the ruling on selling gold to a customer and then keeping the gold as a trust with me?

If the question is about selling gold for currency, then physical possession before parting is a condition, based on the Prophet Muhammad's (peace be upon him) saying: "Gold for silver is usury unless it is hand to hand," and his saying: "Gold for gold, silver for silver... hand to hand."

Paper currency is equivalent to money. The majority of scholars hold that taking possession of gold means taking it by hand, while the Hanafis, Malikis, and a narration from the Hanbalis, permitted "takhliyah" (enabling access) as a form of possession.

So, if the buyer takes possession of the gold by hand, there is no objection to leaving it with the seller. However, reserving the gold by paying a partial value and securing it with the merchant until the full value is paid is not permissible. Rather, the full price must be paid first, and then the buyer has the choice to take the gold or leave it with the seller.