Perhaps fees settle on T when trades settle on T+3. This could result in negative cash balance if you push close to account equity limits.
Thanks, yes I think the fees are charged on trade date vs. settlement. But I think that's shown exactly so on the daily statement, so the net "available funds" should reflect the real thing. I use advisor allocations, and executions at several prices sometimes do not get distributed equally between accounts. IB will square to average prices in the accounts on the following day. Those appear on the following day's statement "as of" the previous day, so there's a possibility of a debit being created that was not seen the previous day. I did not find that to be the case, but I could have missed something. Anyway, no problems since.