What is the meaning of "likeness for likeness" in the Hadith of Sarf (currency exchange), and how can one respond to the misconception that selling gold for the same amount it was bought for is foolish?

If gold is sold for gold, there must be equality in quantity and immediate possession ( قبض ) at the contract session, because the Prophet, peace and blessings be upon him, said: "Do not sell gold for gold except like for like." However, if gold is sold for silver or cash, it is permissible to sell it for more than its purchase price, because the Prophet, peace and blessings be upon him, said: "But if these classes differ, then sell as you wish, provided it is hand to hand."