Samsung a20 faulty

We bought a Samsung a20 6 days ago at foshini sanlam center after we charged the phone the fingerprint sensor didn't want to work and the password on the lock screen. And they gave us a phone thats on display because they didn't have the phone in stock so now i want to know why do they have to send it in for checking and as the phone is faulty.. So why cant we exchanged it for the same one.