Return Policy Issue

I purchased a CD/DVD drive late Oct. I spoke to a Dell Rep that informed me that this particular drive: 16X DVD+/-RW, SATA would be compatible with my Dell Demension 2400. Well come to find out this is not compatible with my computer and because I called a week after the 21 day policy for this product I cannot get my money back even though the Dell Rep was at fault. Does anyone have any suggestions and how I can handle this further? I spoke to Dell and they are refusing to give me my money back. I have a brand new CD/DVD drive and cannot use it. It's unbelievable a company like this is not taking responsiblity for there rep mis-information.