Is it Islamically permissible to raise the profit margin in a new deferred sale to the same customer when he delays in paying a previous installment, especially if prices have risen in the market?

It is permissible to increase the price of an item when selling it in installments. There is no objection to the first sale transaction in which the price was increased in exchange for installments. As for the second sale transaction, the indications suggest that it was conducted at the same price as the first transaction. It is not permissible to increase its price after that due to the buyer's delay in payment, as this falls under the category of prohibited usury (riba).