Is it permissible to give money to a clothes merchant for him to buy merchandise, then he determines a profit margin on installments, and the purchases become the property of the merchant after the purchase?

It is not permissible to give money to a merchant for him to buy goods for himself and then return it with an increase; this is clear usury (riba), and it is one of the gravest major sins. To earn lawful profit, one can purchase the goods and take possession of them, then sell them to the merchant in installments and make a profit from them.