Lucius was still wearing the cold smile. "One hundred thousand Galleons. If you have not that much in your vault, I suppose I must accept a promissory note for the remainder." [...]

"I think there's about forty thousand in my Gringotts vault," Harry said. [...] "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."

Emphasis mine. I think (most of) whatever he has must be payed immediately, and the “due” part is only for the rest.