dear cool79,
if you are so scared abt this trade, why not just do your own hedging by buying fr the mkt tmr? ur loss will be contained and u will be at least be in control of your own fate. square the open buy when the exchange closes your open sell on T+4. your losses should nt spiral out of control unless the price rise too much on buy-in day. btw, buy-in for this kind of shares are usually 99% filled in the morning, so, dun think too much.
trust me, this kind of mistake should not be making you lose sleep. wait till u hear abt the poor bloke who added an extra 0 to his 100,000 shares order.

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