The only issue is: Is the influence card that the Castle owner 'sell/exchange' for 2 Gold RANDOMLY chosen or does the owner decide what influence card to give?
I would say the the owner of the castle decides which card they want to sell.
The text on the castle buildings says:
"...für 1 beliebige Machtkarte 2 Gold nehmen."
which I would translate as:
"...take two gold for one arbitrary influence card."
I hope this helps.