Return or Not Return That is the Question?

bought a desktop for my nephew for christmas from a reputable national dealer. The computer didnt work out of the box. After some trouble shooting the tech department they decided it needed a new hard drive, which they were supposed to send. They ended up sending a whole new computer, monitor and all. I got a call that if the broken computer wasnt returned within 10 days that I would be charged for a new computer. My question is , if I dont send the unsolicited computer back cn they charge my debit card for the new one? I already know the moral answer (send it all back) , whats the legal answer. The hard drive that came with the original just needs windows loaded on it. Can they legaly debit my visa check card for a new one? These are really nice computers. Id like to keep it but dont want to buy a new one