He's got plenty of time, and doesn't need that much seed money or that high a growth rate -- unless Lucius has a plan that'll get intolerable well before his payment comes due. I ran some numbers, and if he's making 3% profit on each of one-third of all trading days for six years (which I think is if anything conservative -- the arbitrage hack early in the story would be a couple orders of magnitude more profitable until someone catches on), he needs a principal of a little under 40 Galleons to break the 100,000 mark by the end of year 7. For 60,000, it's more like 25.

That's a decent amount of money if we're going by the prices we've seen for goods, but I'd be surprised if he couldn't borrow it from any of the suitably impressed adults he's surrounded himself with. Especially if they've got a motive to screw the Malfoys over.