Using money in tracking account eventually

I've got a brokerage account that is currently invested and I've got it in my budget as a tracking account. I have a monthly transaction to spend $800 in "Next New Cars" categories that represents a deposit from my checking account into the brokerage. I think this works well because the car purchase is at least 7 years away, the investment account balance fluctuates because of gains and loss in the market, and I'm able to have an on-budget transaction for the monthly investment. I used to track the brokerage balance as a budget account but that seemed to be the wrong way to handle it because of the gains and losses in the market causing my income and expenses to be skewed.

What I've been thinking about lately is how will I handle when I have to actually make the car purchase? What will I do when I take some of the money out of the investment account and put it in my checking account to make a full cash payment for the car or a down payment on the car?

Count it as income? From who? And then subsequently have a very large expense for the money coming out of the checking account?

Anyone been through this and have advice on how they handled it?