I recently did the same (albeit for $10k). Even after agreeing on everything, they kept asking "are you sure you don't want to finance just part of it? We can give you a really good rate!"
All I could do was patiently ask them "why would I do this? Is it not still more of a cost to me, even if a small one?" They acted really confused and upset but I understood that this was simply a sales tactic to make me feel as if I was making some sort of mistake by not paying some small amount of interest in addition to the cost of the car.
I eventually got a bit irritated as it was making things take a lot longer than they needed to. I just told them, "look, we already negotiated a price and I have the check written out right here. I have no need to finance a $10k car right now and I would like to just finish the sale."
They weren't quite as friendly after that but hey...it's an adversarial relationship and I'd been perfectly cordial up until that point. I was already paying $10k for a car they likely got for half as much as a trade-in anyway.