Question about using instant buying power to make money

So almost all platforms give their members some form of instant buying power, meaning once you iniate a transfer/deposit into the brokerage account you can use X amount of that money to trade with even though the funds have not hit the account yet (and won't for another few days). So my question is let's say someone has a limit of $2,500 in instant buying power and they then 'deposit' 2500 into their account. Only problem is their bank balance is only a few hundred so the transfer won't go thru. However the brokerage account won't know the deposit got declined for a few days. Let's say in the meantime you use the $2,500 in buying power to purchase a stock on Tuesday and Wednesday its up 10%, so you sell it and make $250. You now have $2,500 buying power and $250 actual money in your account. What happens to the $250 you made in a couple days when the deposit doesn't go thru? Will your buying power be brought back to zero and the $250 remain in your account and allowed to be withdrawn? Can they just take that money? Not saying I'm going to do this as I have a somewhat decent portfolio, I've just always been curious as to how firms deal with this and how they protect themselves since anyone could potentially lose money they don't have any after a deposit bounces their account would be negative and the firm loses out. Thanks in advance.