You might be lucky, as I was or maybe this is only a UI bug anyway but when I had this problem a week ago I just collected the 410k gold on a hunch and I was able to buy the house for that regardless. I checked the crown price for the gold just for fun and noticed it was low. I still used a handful of crowns for the last few gold in case it was another bug in itself that allowed me to do this, not sure if it is necessary but try out yourself.