Is it permissible for the association to purchase goods for (800) Riyals and sell them to the customer for (1200) Riyals, provided that the (1200) Riyals are paid in installments?

If the association purchases the aforementioned merchandise and takes legitimate possession of it, then sells it in installments for 1200 Riyals, there is no harm in that, provided the sale adheres to the legitimate Islamic guidelines and is free from deception.