To be fair, this was the first time Hajime volunteered to treat everyone. In the past, he could only wish for such a scenario to happen so now that the situation presented itself to him, Hajime didn't hesitate to take his chance. There was an uncertainty in his voice as he called out his offer, but nevertheless, his offer reached Kichiro and the others' ears while they stopped short on whatever it was they were occupied with. Not only did it sound uncharacteristic for Hajime to say such things, they also couldn't believe that he had that much money on him.
Hajime was unsure whether his offer was the optimal course to take, causing him to avert his eyes and hesitate in an awkward manner in front of his group. But then again, he couldn't think of anything where he'll most likely spend his allowance for the school trip so he figured he'd just treat them.