Purchasing Goods on Instalments

Dr. Monzer Kahf, a prominent economist and counselor, states the following: “Instalments sale is permissible in the Shari`ah even if the price (the total amount of all instalments) is higher than what the seller usually announces as the cash price, whether the difference is expressed percentage wise (cash price + certain percentage) or a given amount added above the cash price.
It is, however, not permitted in the Shari`ah to make the contract optional with two prices: cash and instalments, that is, I sell you this commodity for either a 1000 cash or 1200 on instalments.