Selling the new ticker before the shares are actually deposited in your account make you short those shares. If the company backs out of the roundup your broker will force you to cover. This happened to me when I unknowingly sold the MXSV IB had fronted me. When MXSV refused to honor the roundup IB forced me to cover on the day that they notified me. I lost a little bit because MXSV had gone up in price. Had the price gone down I would have made a profit.
I no longer sell roundup shares until I call the broker for confirmation that the shares are actually in my account.
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