just passing to give you some information,
by rules of the PRO staff, after a auction starts you have no choice to accept the consequences and the last bid on the poke, if you are not happy with only 600k is your problem, you should make a start bid higher, but now any player can report you for fake auction, i dont remember right now what is the punishment for not following with the trade after the end, but its not good, so i hope you dont sell to another person because if the winner report you you can lose a lot more then one poke. you probably will need to follow with the trade.
4. When a deal has been agreed upon, by both parties, you can not go back on it.
This means that from the moment you made the post with start bid at 400k you agree with the terms, you can not go back.