Samsung faulty tab

I bought a samsung Tab3 cash. After switching it on I noticed it was making green lines across the screen on videos and distorted colours after a few minutes of use. Took it back to the cell c branch the next day and after showing the lady working there the problems I was having she said it looks like an OBF (OUT OF BOX FAILURE). She did advice that the device would have to be sent away for testing and to confirm that it wasnt damaged by me (dropping). Today I got the device back after waiting 7DAYS with a report stating that there is nothing wrong with the device and that its functioning correctly. The device is freezing lagging and running green lines across the screen with distorted colours, this is not normal. I have taken screenshots and mailed them to all your contact email addresses.

How can you force me to keep something that is faulty and keep my money after I took it back the next day to tell you there is a problem with it. Ive never returned any Samsung product before but I will also never buy anything from Samsung again. I am really dissapointed in your service.