Y'all can't freeride the market, lol. You have to wait until funds settle before you use them to buy the security, unless you have a cash position large enough to cover the trade.
If you have $100 and you buy $100 worth of Ford, you now have $0. When you sell $100 of Ford, you now have $100. So far, so good?
Now, if you turn around and buy Ford again with any of that money before the previous sell trade settles, that's a no-no. However, if you had additional cash reserves beyond the $100 that you got from your sale of Ford, and you use THOSE to buy Ford, then you're good.
You have to allow time for trades to settle if you're going to use that same cash to move in and out of the security.