"I should have the whole amount ready by the end of the month." It would take his arbitrage trick, but certainly the Headmaster would let him do that instead of going into debt to Malfoy.

Even if he does his arbitrage trick, what benefit would he get from telling it to Malfoy in advance. Why share unnecessary information with a potential adversary? Why risk additional penalties if something unexpected happens and the arbitrage takes five weeks instead of four?