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Can my broker lock my cash account if I try to use the money from a stock sale during the three-day settlement period?
Brokerage firms must settle funds promptly, but there's no explicit definition for this in U.S. federal law. See for example, this article on settling trades in three days. Wikipedia also has a good write-up on T+3. It is common practice, however. It takes approximately three days for the funds to be available to me, in my Canadian brokerage account. That said, the software itself prevents me from using funds which are not available, and I'm rather surprised yours does not. You want to be careful not to be labelled a pattern day trader, if that is not your intention. Others can better fill you in on the consequences of this. I believe it will not apply to you unless you are using a margin account. All but certainly, the terms of service that you agreed to with this brokerage will specify the conditions under which they can lock you out of your account, and when they can charge interest. If they are selling your stock at times you have not authorised (via explicit instruction or via a stop-loss order), you should file a complaint with the S.E.C. and with sufficient documentation. You will need to ensure your cancel-stop-loss order actually went through, though, and the stock was sold anyway. It could simply be that it takes a full business day to cancel such an order.