Having filled their day with other activities after Ron had proposed combining their Gringotts accounts on his day off, they hadn't actually made it to the bank at that point to work out the finer details. It was all providing Hermione went ahead and gave him her consent in creating a joint account of course, which she was yet to do.
But they were there now, heading down to Ron's vault first so he could exchange some of his own money for Muggle money while she was around to assist him. "I already told you why not," Hermione reminded him, her hand gripping the side of the cart. "Because it's not really a joint account if I haven't actually contributed all that much."
As they continued down the horrid path to Ron's vault, she glanced sideways at him. "Why do you look so tense?" Hermione had her suspicions and she didn't really expect him to answer, but it was her roundabout way of telling him to just relax.
Finally they ground to a halt and she gave herself a moment before she thought about climbing out, looking back at Ron as he spoke. "Dinner and a little extra, I think I can manage that. Though if you insist on buying things in the Muggle world all the time, we'll have to get you one of those cards," she teased.