Hammad bin Salama حماد بن سلمة · The Book of Financial Transactions

Muawiyah sold a cup of gold or silver for more than its weight. Abu Ad-Darda' said: "I heard the Messenger of Allah forbid such transactions unless it was like for like."

"The Messenger of Allah forbade us from selling silver for silver unless it was of equal amounts, or selling gold for gold unless it was of equal amounts. The messenger of Allah said: 'Sell gold for silver however you want, and silver for gold however you want."'

the Messenger of Allah forbade selling loyalty or giving it away. (Shah)