Is selling devices by installment with an increase in price (such as selling a device for 1000 Riyals in cash for 1200 Riyals in installments over four months) considered a permissible or forbidden method?

If you sell a commodity for a thousand to someone who wishes to buy it immediately, and for twelve hundred to someone who wishes to buy it deferred, there is no harm. However, if you sell it for a specific price for a specific term and tell the buyer that if he does not pay the price by that term, the profit will increase, then this is precisely the usury of the Age of Ignorance which Islam came to prohibit.