Is it permissible to deposit 5,0 dinars with an iron merchant, on condition of receiving double the amount (10,0 dinars) as profit within six months?

This transaction is forbidden by Islamic law. Its reality is lending a sum of money on the condition that it be returned with a known increase, and this is usury (riba). Ibn Qudamah said: "Every loan in which an increase is stipulated is forbidden, without dispute."