I think, FIFO (First In First Out) if I remember correctly. I guess the only way would be to close the 5k and re-open 2.5k and cover the .9 distribute (23c or something)
Edit: Just Be Sure You have a Market Order ticket filled out and ready to market 2.5k when you click the 5k Close....or Buy in the Asian session, together with 5k you might even move the market and get a better price in your 2.5k
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