Is the transaction described when buying and selling gold by calling and leaving the money or gold with the merchant, with an agreed-upon increase or decrease, and paying a manufacturing fee upon receiving the gold or not paying it when leaving it, contrary to Islamic law?

It is a condition for purchasing gold with money that the exchange takes place in the same session, based on the Hadith (definition) of 'Ubadah ibn al-Samit (may Allah be pleased with him). Monetary currencies are treated as gold and silver in this regard. Therefore, if you leave the money with the seller and authorize him to buy gold on your behalf, then the gold must be received from him, or a proxy other than the seller must receive it. Otherwise, the transaction would be forbidden or a stratagem to delay payment, especially if the seller will mix the money and use it.