Cellphone Problem (Back Button Not Working)

I baught a Samsung I9060I Grand Neo Plus (Black) on 28/01/2016 at Markham (Shop 42, Erf 2635, Cnr Chris Hani & Nelson Mandela Drive, Ellisras, Ext 16) in Lephalale. For some reason the back button on the Samsung stopped working suddenly. The phone is still new and under guarantee so I took the phone back yesterday and reported the problem, and the sales consultant on duty also checked the phone himself and said that it is a Samsung problem. He told me that the phone has to go in for repairs which will take round about 14 days. I told him that in my line of work I cannot be without a phone as my work does not allow this, so I need a replacement so I can go ahead with my daily tasks. He told me that they do not have a replacement phone for me, which I do not think is my problem. The phone is now only one month old and there is no physical damage as inspected and confirmed by the sales consultant, so why can't they just replace my phone for me immediately? I am very unhappy about this because I bought this phone cash also. I did provide him with the warranty and the cash sales slip. If they confirmed this is a Samsung problem and not my fault I think it is more than fare to replace!