I don't think we need to rely so much on your imagination.

"I think there's about forty thousand in my Gringotts vault," Harry said. It was strange how that was still causing more internal pain than the thought of taking an over-fifty-percent risk to his life to destroy Azkaban. "As for the other sixty thousand - what are the rules, exactly?"

"It comes due when you graduate Hogwarts," the old wizard said from high above. "But Lord Malfoy has certain rights over you before then, I fear."

If a debt isn't due, it need not be paid on. There's nothing here that says he's required to pay anything else before he graduates.

In fact, it's possible that the author meant us to understand that HJPEV didn't have to pay anything right now and that the whole hundred grand isn't due until graduation. But that would mean the author had been a bit sloppier than I'm fairly confident he is.