Purchasing an item on credit from Makro

A person wishes to purchase a stove on credit from Makro (for example). The cash price of the stove is R5000. However, Makro says that if you wish to purchase it on credit, there will be a certain percentage interest rate. Hence purchasing it on credit will cost R5800 over 20 months at R290 per month. Makro says that the installment will NOT fluctuate during the months. Every month it will certainly remain at R290 till it's fully paid. When he purchased it, he ignored completely what they had to say about the 'interest rate', and his intention was that they are merely changing the price of the stove and just making more profit on him since he's buying it on credit. Is he guilty of ribaa in the above mentioned case?