Allocating Money in a Brokerage Account

I have a fairly large purchase coming up in the next 6 months. I was thinking of creating a "property & debt" item that represents that purchase. Then enter a transfer from my fidelity brokerage account into that. But I'm guessing every time I do an Update I'll get an error that my cash balance does not match the on-line balance. The only way I can think of is to open a new account with Fidelity and move that money there for safe keeping. Otherwise I would need a post note saying "Don't spend $xxxxxx.xx dollars!" :-)