Upgrade gone seriously worng

I recently upgraded my mtn contract with an Xperia tab Z on a 500mb p.m .When i recieved the device it has a factory fault, took it back to the branch and got it exchanged however the second device also had the same fault, the device was then sent for a warrenty check. When i went in store to query about my device one of the employee's Sanele said that the device was damaged and i will be credited for amount to choose another device, with knowing this i was told i have 31 days to choose the device i went in store again to take another device, The manager says thats not how mtn works and i have to take a cheaper device beacuse they dont offer the contract for the tabZ anymore, i have now payed 2 months of the contract without any device, i was told the premiums where R249.00p.m yet im charged R494.99p.m , i wish to cancel the contract because i am very unhappy of being misled by the employees and the attitude of the manager when i confronted him for help.