I wonder if the firm charges you Interest on Intra Day margin/buying power? I think in bank world, there is no Intra day interest. So you can borrow from morning then pay back in afternoon. example 1, i have 10k in bank of america with over draft protetion of 10k, i wrote a check of 15k. the money is out on friday morning, then i go to back to deposit a 5k cash on friday afternoon. there is no interest on the 5k i borrowed for half day. However, if there is no interest, why on earth somebody will lend you Interest free money? well they must keep a capital pool for long time which cost them interest, unless their capital pool is also intra day only from somebody else, - like the pro trading firm borrow from banks at 8am then pay back at 5pm.