Is it permissible to exchange gold for other gold that is heavier and more beautiful, paying the difference in cash in the same sitting? And what is the solution if it is not permissible? And how can the perpetrator repent from this sin after approximately 3 months have passed?

Selling gold for gold with an increase due to craftsmanship is prohibited according to the majority of scholars, and this is the correct view, based on the Prophet’s (peace be upon him) saying: "Gold for gold, like for like, equal for equal, hand to hand," and his saying: "Whoever gives more or asks for more has engaged in usury." The correct way is to sell the old gold first, and then buy the new gold with its price. Whoever engages in this prohibited transaction must repent and annul the contract if possible; otherwise, sincere repentance suffices him.