It is lawful to buy a thing in less price in case of cash and in high price in case of debt (on instalments), but the price should be fixed in advance. For example, I want to buy the car on debt then the seller said that it shall be available in 15 thousand then this is lawful. Or the buyer said that I want to buy the car in cash then the seller said it shall be available in 10 thousand then also the dealing is correct. But if the seller said that if you would buy it in cash then it shall cost 10 thousand and if you buy it on instalment then 15 thousand, so this dealing is not correct; since any particular mode is not fixed.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband