Dell complaint, asking for invoice

I had a problem with my system for which I reported to Dell. They replied that they need my system invoice for evaluation. I told them that I didn't have the invoice. But that I had my warranty. They asked me for my warranty invoice which I send them. 8 days after that they replied to me telling me that they need system invoice and the warranty invoice wont apply. I have already used Dell support 2 times before but they didn't ask for invoice that time but now they are. Why do they need the system invoice when I already have the warranty invoice? If I needed the system invoice they should have told beforehand giving us the warranty only. They are just delaying us and not answering in the best way possible. Now they are just fixed on asking us for our system invoice knowing that we don't have it and wont move on any further with our request.