Neville hadn't really been rejecting Draco's money so much as ensuring he didn't have to go back inside. Still, he was surprised upon exiting the store to have money immediately thrust upon him. It was far too much and Neville was about to protest until he saw Draco's expression and thought better of it.
"Yeah, anytime," he said, putting his hands in his pockets. "Do you want a drink or something? We can sit outside the Hog's Head. It's usually not crowded during the day. Or if you want to get back to your shopping, I understand." He wasn't sure if Draco still wanted company or if he just wanted Neville to leave him alone, so Neville provided him with both options without asking directly.