Imam Malik, may Allah have mercy on him, said that if gold is bought in exchange for silver or silver in exchange for gold in heaps, there is no objection, whether they are ingots or jewelry. However, buying rupees for ashrafis without counting is not permissible; rather, there is deception in it, and it is against the custom of the Muslims. But buying gold or silver ingots or jewelry that are sold by weight, to purchase them by estimation, just as wheat or dates, etc., are bought, is not objectionable.
Imam Malik, may Allah have mercy on him, said that if a person buys the Noble Qur'an, or a sword, or a ring in which there is gold or silver, in exchange for rupees or gold coins, then it should be seen: if there is gold attached to these things and it is bought in exchange for gold coins, and the price of that item is not less than two-thirds, and the value of the gold attached to it is not more than one-third, then it is permissible as long as it is a cash-for-cash transaction. Similarly, if there is silver attached and it is bought in exchange for rupees, then the same ruling applies.
Hadith Referenceموطا امام مالك رواية يحييٰ / كتاب البيوع / 1339