What is the ruling of Islamic law concerning the work of a goldsmith who reduces 7% of the weight of the gold given to him for crafting, knowing that this reduction actually occurs during the work process?

What you are doing by reducing the weight of gold pieces is outright deception and theft, which is forbidden. This is based on the Prophet, peace and blessings be upon him, who said: "Whoever deceives us is not of us," and "No thief steals while he is a believer." The reduction in the piece's weight during work is not an excuse because it is not intended [by the owner], and the owner has the right to claim whatever falls off it. Repent to Allah from this act, resolve not to return to it, and return the rights to their owners if you know them or to their heirs. If you cannot find them, then give the equivalent in charity for the general welfare of Muslims on behalf of their owners.