For example you choose a farm and a ship to build and you put them in your construction list
http://img329.imageshack.us/img329/2885/testeu1.jpgnow you select to buy the farm and you will get the confirmation box how much you will pay for this farm etc
you are still able to remove items on the construction list while the buy window is open
so if you remove your farm by dobble click on the construction list, you will pay for the ship the same credits and resources like for the farm
http://img113.imageshack.us/img113/9510/test2ch1.jpgwell i'm playing the german version but i think this is a general bug and not only in the german version